There are a great many religions in the world today that claim to have a Christian foundation, but specifically reject the Trinity as a valid Biblical concept. What's funny is that while they're all viciously opposed to each other, they all borrow each other's arguments. This ultimately means that the ability to defend the Holy Trinity is a very powerful weapon in the apologist's arsenal, as it single handedly defeats a plethora of false religions. These religions include, but are not limited to, Islam, Mormonism and The Watchtower Society.
Defending the Trinity is relatively simple. For sure, there are several Bible verses one can misquote in an attempt to disprove it, but for all the attempts to refute the doctrine of the Trinity, it is an inescapable conclusion from reading the scriptures as a whole. In fact, as you can see by the above image, an unbreakable defence of the Trinity can be summarised in a single meme.
The first step to proving the Trinity is the fact that the Bible repeatedly affirms that there is one God. There are not two gods, or three, or many more than that, but one. Most significant are very strong statements in the book of Isaiah, in which God declares He does not know of any other gods (Isaiah 44:6-8), and that no gods were formed before Him, nor shall there be after Him (Isaiah 43:10). Realistically, this should be Christianity 101. Even a cursory glance at Christianity, which most Westerners experience during their mandatory school attendance as part of the curriculum, will expose the learner to the monotheistic nature of Christianity.
The second step in proving the Trinity is establishing who God is. If only one being is identified as God, whereas other traditional members of the Trinity are not, that means the Trinity is a false doctrine. If, for example, the Bible preaches that the Father is God, but that Jesus is just a man, and the Holy Spirit is nothing more than an unconscious being, the doctrine of the Trinity is false because it teaches that both Jesus and the Holy Spirit are God.
There is no real dispute (except in Islam) that the Father is God. Even in Islam, the Biblical attributes ascribed to Allah mean he is effectively intended to be the Islamic equivalent of the Father. Yes, Islam explicitly teaches that Allah is not a Father, but Allah is identical to the Father in most other aspects. I could prove that the Father is God, but the absence of dispute of this fact means I don't really need to. But what of the Son and the Holy Spirit? Are they identified as God?
There are three ways in which the Son and the Holy Spirit are identified as God. The first is through their own words, which applies more to Jesus than the Holy Spirit (I don't even recall a single instance of the Holy Spirit directly saying a word). Jesus stated many things, which I will discuss in a moment, in which He either claims to be God, or makes a claim that only God can make. The second way is to be identified by a witness, whether this witness be a sympathetic individual, such as Thomas, or a hostile witness, such as the Pharisees (again, this will be discussed in a moment). Interestingly, God Himself identifies the Son as God. The third way is to be identified by exposition by a Bible author.
As I just said, I am unaware of any time in the Bible when the Holy Spirit speaks directly. It could be in there, but being a fallible human being with a weak memory, I am not aware of it. Jesus, on the other hand, was not afraid to let the world know who He was. The first example is in John 10:30, in which Jesus states "I and my Father are one". The previous few verses also identify Jesus as being one with the Father even without verse 30, as Jesus first claims no one can snatch His sheep from His hands, and then claims this is because no one can snatch them from the Father's hands. This claim would be enough without verse 30, but in the light of verse 30, Jesus clearly identifies Himself as God.
Another clear example can be found in John 14. In verses 7-11, we find an account of Philip and Jesus speaking. Philip asks Jesus to show him the Father, to which Jesus replies "Have I been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father; and how sayest thou then, Show us the Father?" I remember distinctly a Jehovah's Witness attempting to beat me to the punch on this one. In his attempt to evangelise me, he cited the verse and tried to build an analogy. He asked me how I would respond if someone asked "are you the son of (insert my dad's name here)", which of course I would answer in the affirmative. But that's not what the verse is saying! If someone asked me to show them to any one of my relatives, father or otherwise, my response would not be "if you've seen me, you've seen my dad." I would tell them that my dad lives more than 300 miles away. Similarly, unless Jesus really was God in flesh, He would have been lying when He said "He who has seen Me has seen the Father".
Aside from direct claims to be God, Jesus also made claims that He certainly could not make if He was a mere mortal. Only God could make a claim like "before Abraham was, I am" (John 8:58). In English, this is impressive enough. "Before Abraham was, I am" is a very eternal claim, putting the present tense before the past. In other words, "I'm timeless. I'm sitting here before you long after Abraham died, and at the same time I'm living before Abraham." Not the kind of claim a man in his early 30s can legitimately claim without some form of prior existence. But the English is only half the story. In using this particular phrase, Jesus was identifying Himself as God by claiming His very name! When God first approached Moses, Moses didn't know Him as well as we do today. He knew God by only one identity: "The God of your fathers". So he asked what God's name is, and God's response: "I Am That I Am: and he said, Thus shalt thou say unto the children of Israel, I Am hath sent me unto you.’" (Exodus 3:13-14).
We see, then, that Jesus both claimed implicitly and explicitly to be God, but He isn't the only one in scripture to do so. We see that both friendly and hostile witnesses both identified Jesus as God, or at the very least as claiming to be. We have already looked at John 10:30 as an example of Jesus claiming to be God, but further proof that this is a claim to be God comes from the reaction of the Jews. They immediately picked up stones with intent to kill Jesus with them, to which Jesus responded by asking which of His good works they sought to stone Him for. The Jews replied that they were not seeking to kill Him for His good works, but because He claimed to be God.
An example of a less hostile witness to Christ's divinity is Thomas who, upon seeing the risen Lord, exclaimed "My Lord and my God." (John 20:28). Jesus' response is not one of correction, but affirmation. Not "how dare you take the Lord's name in vain", or "uh, no, I'm just a prophet", but more of a "finally you believe! Why did you have to see it first, you utter plonker?"
So here we have Jesus claiming to be God, a disciple confessing that He is God, and the Pharisees being all too aware that Jesus claimed to be God, but it's interesting to note that God Himself calls Jesus God! Speaking of God, Hebrews 1:8 says "But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom." What does God say to the Son? "Thy throne, O God, is for ever and ever". One more time: "Thy throne, O GOD, is for ever and ever". Now, if God, who will share His glory with no man (Isaiah 42:8), calls Jesus "O God", and says His throne is forever and ever, what does that make Jesus? God!
So we see that Jesus claimed to be God, His contemporaries at the very least acknowledged His claims to be God, and His disciples knew full well He is God. Even God Himself claimed that Jesus is God. But finally, in favor of Jesus being God, there is the fact of divinely inspired exposition. Several places in the Bible identify Jesus as God, most famously the first chapter of John. In it, we read In the beginning was the Word, and the Word was with God, and the Word was God. This first verse identifies the Word first as a distinct being from God, yet being simultaneously synonymous with God.
Reading on a little further in verses 14-18, we see undeniably that the Word that was with God in the beginning, and actually was God, is none other than Jesus. Clearly John believed that Jesus is God, and saw fit (under inspiration of God, of course) to include this fact in the opening of his Gospel.
Another example is in Acts 20:28, in which we are told that God purchased the Church with His own blood. The Father, being spirit, has no blood. The only blood God has is in Jesus, God incarnate. It was He who purchased the Church with His blood.
With all of the above information, we see that it is Biblically impossible to escape the conclusion that Jesus is God. Jesus identified Himself as God. Jesus' opponents recognised that He called Himself God. Jesus' friends identified and worshipped Him as their God (and He accepts this). The Bible writers explained that Jesus is God. Even the Father, whom no one disputes is God, identifies Jesus as God. Therefore, at the very least, we have indisputably proven the existence of a "binity". But what of the Holy Spirit? Let's cover Him now.
Before we establish whether or not the Holy Spirit is God, we must first dispel the myth that He is just a force, power, or extension of the Father. Anti-Trinitarians frequently reject the idea that the Holy Spirit is a conscious being, with His own thoughts, feeling and gender. They refer to Him as an "it", and detest all reference to Him as actually being an individual. And yet, this never happens in the Bible. Not even once. Read it once, twice, backwards or upside down, 100% of references in the Bible to the Holy Spirit describe Him as a He. For example, when Jesus promises the disciples that He will send a helper (that helper being the Holy Spirit), He says that "he shall testify of me" (John 15:26). We see in this verse that He is sent by the Son, and thus He must be distinct from the Son. We also see that He proceeds from the Father, so He must be distinct from the Father. The Holy spirit, whether He be God or not, is clearly a distinct, personal being that is neither the Father, nor the Son. The question becomes, therefore, is He God?
The answer to this question is absolutely yes, the Holy Spirit is God. Though, as I have stated, I am not aware of the Spirit directly identifying Himself as God (though as the effective author of scripture, as 2 Peter 1:21 says, all references to the Holy Spirit being God come from the Holy Spirit), I am aware of at least one instance in which Paul identifies the Holy Spirit as God. In Acts 5:3-4, Paul chastises Ananias for lying about the price of a property. Paul claims that Ananias has lied to the Holy Spirit, and in doing so has not lied to man, but to God Himself. The result is that Ananias, and his wife, who told the same lie, are both killed by the Holy Spirit Himself.
Further evidence of the Holy Spirit's Godhood comes from the Great Commission, in which Christians are commanded to baptise in the name (singular) of the Father and of the Son and of the Holy Spirit. In this phrase, found in Matthew 28:19, Jesus elevates the Holy Spirit to the level of God in a move matched only by His own claim to be the great "I AM" of the Old Testament.
The most conclusive proof of the Holy Spirit's Godhood can be found in two places in scripture. In both Mark 3:29 and Luke 12:10, Jesus gives a stern warning: You can blaspheme the Son and be forgiven, but you cannot blaspheme the Holy Spirit and be forgiven. As regicide is a crime that can only be committed against a queen, blasphemy, by its very definition, is a sin that can only be committed against God. Therefore, if blasphemy against the Holy Spirit is such a serious sin that it is even worse than blasphemy against the Son, it logically follows that the Holy Spirit is God!
In this article, which is by no means extensive, we have seen that the Bible inescapably teaches the doctrine of the Trinity. The Bible repeatedly affirms that there is only one God, a fact so well known that one need not have even read the Bible to understand that Christianity teaches it. We've seen that it identifies three independent figures as God. The Father, without dispute, is seen as the God who created the heavens and the earth. The Son identifies Himself as God, has His claims to be God recognised by His enemies, is worshipped as God by His friends (which He does not resist or reject), and is identified as God by divinely inspired exposition. The Father also identifies the Son as God, testifying that His throne will last forever, and the Holy Spirit, having inspired the very scriptures that describe all of these things, identifies the Son as God by proxy. We have thoroughly refuted the myth that the Holy Spirit is nothing more than an impersonal force, and does indeed exist as a distinct being with His own mind. The Holy Spirit identifies Himself as God through the scriptures, is identified as God by Paul, can be blasphemed as God, and is identified as God by Jesus' command to baptise in His name. Anyone who claims that the doctrine of the Trinity is anything other than a well established Biblical fact has either not read the Bible, has not retained what they have read in the Bible, or is simply flat out lying.