Christianity and the doctrine of the Trinity are inseparable. The Trinity is perhaps the most well-known Biblical doctrine, even outside the Christian faith, and is notably a uniquely Christian doctrine. No other religion has this, and some are explicitly set up to deny this. Even Judaism, while it does have a history of primitive Trinitarianism (of course, due to its heavy reliance on the Old Testament) has largely distanced itself from the multi-personal God.
But is believing the Trinity a requirement for salvation? The answer is a little more complex than most people would like, simply because the answer is "yes, but...". See, the great thing about Christianity is the simplicity of it. We do not have to be expert theologians to be saved, and we can even be horribly wrong in some areas while still remaining in God's grace.
One of the greatest examples of salvation in recorded history is the penitent thief. When Jesus was crucified, He was not crucified alone. Rather, on either side of His cross, there were two thieves, one who mocked Him, but the other who repented. In Luke 23:39-42, we read "Then one of the criminals who were hanged blasphemed Him, saying, “If You are the Christ, save Yourself and us.” But the other, answering, rebuked him, saying, “Do you not even fear God, seeing you are under the same condemnation? And we indeed justly, for we receive the due reward of our deeds; but this Man has done nothing wrong.” Then he said to Jesus, “Lord, remember me when You come into Your kingdom.” And Jesus said to him, “Assuredly, I say to you, today you will be with Me in Paradise.”"
From this, we see just how easy it is to be saved. You don't need to be baptised, or receive communion. You don't need to be confirmed, either as a member of a specific denomination, or as a Christian in general. You don't need to say the famous "sinner's prayer". You don't need to speak in tongues, or attend a church service. You don't even need a full understanding of any complex doctrine. On the cross, the penitent thief did nothing more than confess, and repent of, his sin ("we indeed justly, for we receive the due reward of our deeds") and confess Jesus as Lord, whom He evidently believed would rise again ("“Lord, remember me when You come into Your kingdom.”").
When it comes to the necessity of works in salvation, the penitent thief is the ultimate trump card. Is baptism essential for salvation? No, penitent thief. Is going to church essential for salvation? No, penitent thief. Is speaking in tongues essential for salvation? No, penitent thief. When it comes to doctrine, however, things get a little more complicated.
We can begin with the fact that the Trinity does not seem to be immediately in view here. The penitent thief rightly called Jesus Lord, but there is no indication that he knew exactly what that meant. I quite like Alistair Begg's imaginary description of this man in Heaven. "That’s what the angel must have said, you know: “What are you doing here?” “I don’t know.” “What do you mean you don’t know?” “I don’t know.” “Excuse me, let me get my supervisor.” They go get their supervisor angel. “So, just a few questions for you. First of all, are you clear on the doctrine of justification by faith?” The guy said, “I’ve never heard of it in my life.” “And what about the doctrine of Scripture?” This guy’s just staring — and eventually, in frustration, [the supervisor] says, “On what basis are you here?” And he said, “The Man on the middle cross said I can come.”"
Now, ultimately, that is all that our salvation comes down to. You can do, say, even think, whatever you will, if "the Man on the middle cross" says "come in", in you go, and if He says "I never knew you, depart from me you worker of iniquity", you're off to the lake of fire, where there will be weeping and gnashing of teeth. What's even more amazing is that Jesus effectively compares all this to the mind of a child. "Let the little children come to Me, and do not forbid them; for of such is the kingdom of God." (Matthew 19:14; Mark 10:14; Luke 18:16). Jesus even says "Assuredly, I say to you,unless you are converted and become as little children, you will by no means enter the kingdom of heaven." (Matthew 18:3).
Now, is a child going to know much advanced doctrine? A small enough child may not even know the words "advanced" or "doctrine". So we can correctly say that even one as uneducated as the penitent thief may nevertheless be saved, or we can erroneously assert that a man must be able to affirm and recite various creeds and confessions by heart.
With that in mind, the doctrine of the Trinity is undoubtedly one of the most complex doctrines Christianity ever produced. In fact, I dare say that even the most advanced theologians will, ultimately, consider it above their paygrade. So there is one God, yet three coequal individuals are that one God, united in essence, yet separate in thought? I don't think we're ever going to get our heads around it, much less find a perfect way to explain it in English. But then how can I say "yes, but..."? If the Trinity is so essential to salvation that a Trinity denier cannot be saved, how can one also say, in the same breath, that we, who do not understand the Trinity, are nevertheless saved because "the Man on the middle cross said I could come"?
The answer is that there is a difference between faith and understanding. As Proverbs 3:5-6 tells us, "Trust in the Lord with all your heart, And lean not on your own understanding; In all your ways acknowledge Him, And He shall direct your paths." And we see this exemplified time and time again throughout Scripture. The Apostles didn't always understand Jesus. Take, for example, Mark 9:9-10: "Now as they came down from the mountain, He commanded them that they should tell no one the things they had seen, till the Son of Man had risen from the dead. So they kept this word to themselves, questioning what the rising from the dead meant." The Disciples were true disciples. With the sole exception of Judas, who was a devil from the beginning (John 6:70), every one of them was ultimately saved. But there was a time, early in their walk, when they did not know how Jesus was actually going to save us. All they knew is they wanted to follow Him. They had nowhere else to go, because they knew that Jesus had the words of eternal life.
And of course, we look further back in time, before the virgin birth even, and we see that people were still being saved, completely apart from the more advanced Christian doctrine. They were still saved by faith, but they didn't know by faith in exactly what! As we read in 1 Peter 1:10-12, "Of this salvation the prophets have inquired and searched carefully, who prophesied of the grace that would come to you, searching what, or what manner of time, the Spirit of Christ who was in them was indicating when He testified beforehand the sufferings of Christ and the glories that would follow. To them it was revealed that, not to themselves, but to us they were ministering the things which now have been reported to you through those who have preached the gospel to you by the Holy Spirit sent from heaven—things which angels desire to look into."
Thus, we see that they worshiped the same God, and were saved by essentially this same Gospel, but they didn't actually know this. They foretold, in the Holy Spirit, things which they did not yet understand, but they never actually saw it on this side of the void. Jesus tells us similar things in Matthew 13:16-17 and Luke 10:23-24.
Ultimately, this all culminates in Galatians 3:22-25, which tells us "But the Scripture has confined all under sin, that the promise by faith in Jesus Christ might be given to those who believe. But before faith came, we were kept under guard by the law, kept for the faith which would afterward be revealed. Therefore the law was our tutor to bring us to Christ, that we might be justified by faith. But after faith has come, we are no longer under a tutor." Revelation, you see, was gradual. Adam and Eve never owned a Bible. Noah didn't even have access to Genesis 1. Abraham never had the Torah. Moses never read the book of Malachi. Even the earliest Church did not have the New Testament. Traditionally, though I personally find this date to be unrealistically late, the book of Revelation was not published until 95 A.D. That means even Christ's most famous Apostles, with the sole exception of John, did not possess a complete New Testament!
But evidently, God didn't desire us to stay in the former ignorance. Indeed, this sort of attitude is thoroughly condemned in Scripture. We are told "Brethren, do not be children in understanding; however, in malice be babes, but in understanding be mature." (1 Corinthians 14:20). Note, that's two exhortations to be mature in understanding, but only one to be babes with regard to malice. Understanding, therefore, would appear to be more important even than avoiding sin.
This shows us that even though we can, theoretically, be saved without understanding theology, we should nevertheless seek to. "Not essential for salvation" and "not essential at all" are two very different things.
But Scripture shows us the necessity of the Trinity for salvation in several ways, starting with the Deity of Christ. In John 8:24, we read "Therefore I said to you that you will die in your sins; for if you do not believe that I am He, you will die in your sins.”" This is a big deal. This is something Jesus is setting forward as an essential belief. If you do not believe this one thing, you will die in your sins. And what is this one thing? "egō eimi" (ἐγώ εἰμι).
On its own, these words mean very little. It's literally just "I am". I am what? I am a man, I am a Christian, I am writing an article on the Trinity. Christ Himself uses these words quite often, particularly in the book of John. "I am the bread of life." (John 6:35). "I am the light of the world." (John 8:12). "I am the true vine" (John 15:1).
But there is something more to this otherwise simple pair of words "egō eimi". In John 8:24, most modern translations render it "I am He". But where does this "He" come from? In truth, it's added. There is no "He" in this verse. So why is it in our modern translations? Simply because an unqualified "egō eimi" is "bad Greek". That is, it is effectively meaningless, as indeed it is in English. Walk up to someone and say "I am", what are they going to think? They may wait for you to complete the sentence, or perhaps they will complete it for you. "Weird, that's what you are". But here, Christ says unless you believe "egō eimi", you will die in your sins.
So what's He saying? Put simply, He's ascribing the Divine name to Himself. In Exodus 3:13-14, we read "Then Moses said to God, “Indeed, when I come to the children of Israel and say to them, ‘The God of your fathers has sent me to you,’ and they say to me, ‘What is His name?’ what shall I say to them?” And God said to Moses, “I AM WHO I AM.” And He said, “Thus you shall say to the children of Israel, ‘I AM has sent me to you.’ ”"
So who sent Moses? "I AM". Who must we believe Jesus is to be saved? "I Am". To really drive this point home, He actually doubles down on this towards the end of the chapter. "Your father Abraham rejoiced at the thought of seeing my day; he saw it and was glad.” “You are not yet fifty years old,” they said to him, “and you have seen Abraham!” “Very truly I tell you,” Jesus answered, “before Abraham was born, I am!” At this, they picked up stones to stone him, but Jesus hid himself, slipping away from the temple grounds." (John 8:56-59).
Why did they try to stone Him? While it doesn't explicitly say it this time, there is no reasonable doubt that they knew He was calling Himself God. In fact, there is nigh universal agreement among Bible scholars that John 8:58 is an example of Jesus ascribing the divine name to Himself. Yet, there are no sound reasons for distinguishing the "I Am" statement of verse 24 from that of verse 58.
This verse alone shows that the Deity of Christ, at the very least, is essential for salvation. Of course, there are more, particularly those which compel us to confess Him as Lord in order to be saved. The meaning of that word "Lord", being the very criteria for our salvation, matters. If you don't confess Him as Lord, you are not saved, yet that word "Lord", when it comes to Christ, is indistinguishable from God. Now, this article is not designed to establish the truth of the Trinity, but its necessity for salvation. Thus, I will not be defending that particular statement here. However, the Defending the Trinity section does contain other articles which establish the Deity of Christ.
So, evidently the Deity of Christ is essential for salvation. If you do not believe Christ when He says "I am He", you will die in your sins. But there is a distinction between the Deity of Christ and the Trinity as a whole. Does one have to believe the Holy Spirit is God in order to be saved? (I take it for granted that one must believe in the Father).
The necessity of the Holy Spirit is slightly harder to establish. To my knowledge, no verse or passage says anything along the lines of "if you confess not the Holy Spirit as Lord, thou art destined to destruction". But there are Scriptures, as well as Biblical principles, that make this concept clear.
To begin with, one would have to ask how, given that the Trinity is one in essence, one could possibly be saved in spite of one member. We all know it's impossible to reject Christ without rejecting the salvation He brings. And obviously, since Christ saves by the power of the Father, one cannot reject Him either. Yet, somehow, we are able to reject the Holy Spirit, yet retain Him as the seal of our salvation (e.g. Ephesians 1:13-14)?
But Scripture does show the severity of blaspheming the Holy Spirit. At this point, many students of Scripture may anticipate, and pre-emptively reject, the example I am about to bring up, but bear with me. I am, of course, speaking about the infamous "unpardonable sin". Twice in Scripture, once in Matthew 12:22–32, and again in Mark 3:22–30, we read about this concept. Now, as I've said before, there is an element of this particular sin that is no longer available to us today. The context simply precludes this as a possibility. Jesus is no longer on this Earth performing miracles, and so we cannot behold these miracles and say "He has an unclean spirit".
But this is similar to saying that we can no longer "lay a hand on the Lord's anointed", in the sense of 1 Samuel 24:10. The act is no longer possible, because Saul has long since died, but the concept is still there. Because the concept is still very much real, though the act is impossible, we can still learn something.
In much the same way, Matthew and Mark do not record the unforgivable sin simply for posterity's sake. Rather, it shows us the Holy Spirit is every bit as much God as the Father and the Son. Thus, while you can no longer blaspheme Him in the unforgivable way, blasphemy against Him is, in all the same ways, a sin.
Now, what sort of ways can you blaspheme? You can deny the existence of God. You can use His name in unclean ways. You can accuse Him of doing evil. These things can be done to every single member of the Godhead. You can blaspheme the Father, the Son, and the Holy Spirit. These are all sins for which we can be forgiven, in Christ. But just as dividing the Father, or the Son, from the Godhead, is antithetical to salvation, so also is the removal of the Holy Spirit.
So antithetical is this from salvation that, throughout Scripture, we actually see the Holy Spirit working in salvation just as intimately as the other two members of the Godhead. We see, for example, in 1 Corinthians 12:3: "Therefore I want you to know that no one who is speaking by the Spirit of God says, “Jesus be cursed,” and no one can say, “Jesus is Lord,” except by the Holy Spirit." So wait, you can only say "Jesus is Lord" by the Holy Spirit, yet that same Holy Spirit will not compel you to confess Him also? As if you're going to enter the Kingdom of Heaven, finally seeing the Father, and the Son, and then "oops, sorry Holy Spirit, I hope we're cool now...".
I think, ultimately, the crux of this issue all comes down to the level of spiritual maturity a particular person has. Scripture does make it quite clear that we are responsible for whatever revelation God has given us. As Christ says in the parable of the good and evil stewards, "And that servant who knew his master’s will, and did not prepare himself or do according to his will, shall be beaten with many stripes. But he who did not know, yet committed things deserving of stripes, shall be beaten with few. For everyone to whom much is given, from him much will be required; and to whom much has been committed, of him they will ask the more." (Luke 12:47-48). In a similar vein, He was quite clear that even those who are condemned, ultimately, will only be punished according to their level of revelation. "Then He began to rebuke the cities in which most of His mighty works had been done, because they did not repent: “Woe to you, Chorazin! Woe to you, Bethsaida! For if the mighty works which were done in you had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes. But I say to you,it will be more tolerable for Tyre and Sidon in the day of judgment than for you. And you, Capernaum,who are exalted to heaven, will be brought down to Hades; for if the mighty works which were done in you had been done in Sodom, it would have remained until this day. But I say to you that it shall be more tolerable for the land of Sodom in the day of judgment than for you.”" (Matthew 11:20-24).
So with that in mind, the charity of God becomes evident. You can have the theological understanding of a child, who may still say something as inaccurate as "God and Jesus", not even knowing the Holy Spirit exists, and you may still be saved. At the same time, we are expected to grow, and that growth will, inevitably, result in discovering the Trinity. The result of this discovery will require the acceptance thereof.
We find a similar concept in John 9:41: "Jesus said to them, “If you were blind, you would have no sin; but now you say, ‘We see.’ Therefore your sin remains." Because of Christ's finished work on the cross, we are not responsible for what we don't know. But just as the Jews, who might have been saved not knowing Christ, became accountable to Him when He came, so also is a man who, now knowing the Trinity, required to accept it. Therefore, there is a sense in which the Trinity is not necessary for salvation, but also a sense in which it is.