When it comes to the possibility of losing salvation, there are two main views: Conditional Security (the belief that salvation can be lost), and Eternal Security, a.k.a. the Perseverance of the Saints, which is often simplistically summed up as "once saved, always saved".
When asking the question as to whether or not salvation can be lost, the ideal starting point would be "if we can, how?" There are realistically only two ways one could lose salvation. The first is to tick God off to the point where He considers you unworthy, and the second is to lose the faith that activates His saving grace.
Believers in Conditional Security vary in their answers to this. Of course, they'll never give a straight answer, because in reality, neither does the Bible. The Bible literally never says "this thing results in the loss of salvation." Thus, any attempt to use scripture to justify the loss of salvation must necessarily be inferred from a less explicit verse. And that is absolutely fine. No solid Christian would ever condemn inferences from the scriptures. Theology is all but entirely built on inferences. For sure, there are instances where doctrines are explicitly stated. Creationism, for example. The Bible explicitly tells us that the Lord created the heavens and the earth in 6 days (Exodus 20:11, see also Genesis 1). But then you come to doctrines such as the Trinity, which is logically inferred from the fact that three separate beings are all referred to as one single God.
So, clearly, inferences are not necessarily a bad thing. However, barring logical necessity (e.g. if I say "today is Monday", it necessarily follows that tomorrow is Tuesday), inferences must be noted as being significantly weaker than explicit statements. We must keep this in mind when we hear inferences that salvation can be lost based on verses like Hebrews 10:26. Hebrews 10:26 is a difficult verse to wrestle with if you've never studied the issue, however it does not explicitly say that salvation can be lost through willful sin. Rather, this is a conclusion drawn from casually reading a single Bible verse, which is bad practice at the best of times.
Putting Hebrews 10:26 in our back pocket for a moment, the rest of scripture contains a very consistent message about salvation. Specifically, how it is received. Note how I said received, rather than earned. This is because scripture is also consistent in its message that it can't be earned. At least, not by us. We are constantly told that all have sinned and fallen short of the glory of God (Romans 3:23), that the wages of sin is death (Romans 6:23), that our good works are filthy rags (Isaiah 64:6), that any sin makes you guilty of violating the whole law (James 2:10), and that you can even call Jesus "Lord" before boasting of your good works, and He may still say "I never knew you, depart from me you who practice iniquity" (Matthew 7:21-23).
By contrast, salvation is always presented as a gift (Romans 5:15-18, 6:23; Ephesians 2:8), offered by the grace of God, and received by faith, apart from works (Ephesians 2:8-9), which actually leads to good works (Ephesians 2:10). What all of this tells us, both explicitly and by inference, is that works are not a part of salvation. Thus, there is literally no way to earn, or lose, salvation by works.
But let's suppose you could. Let us temporarily set aside the explicit statements that say salvation is obtained only through faith, and that works and grace are literally opposites (Romans 11:6), so you can't be saved by both. If we remove those statements (which is, of course, extremely bad), we still have to wrestle with the fact that it's the same Holy God we're dealing with here. He doesn't gain more tolerance for sin when we confess Jesus. Thus, if sin could lose you salvation, any sin could lose you salvation. But if we say we're without sin, we're delusional, and we're actually calling God a liar (1 John 1:8-10). If you think murder is bad, good. You're right. If you think adultery is bad, good, you're right. But if you think a white lie, or a cuss word, or gossip, or laziness, or petty theft, are "no big deal", you're forgetting that if Christ had not been crucified, every single one of those "small" sins, and more, are so unbelievably repugnant to God that He would literally send you to Hell for them forever! How much more serious do you think it is to Him after He spent 33 years on the Earth and died on a cross?
But that's where Conditional Security advocates will say "yeah, but it's only willful sin". Problem 1: The same applies. If you willfully tell a white lie, that's still bad. Problem 2: That's still works based salvation. If you have true faith, even willful sin can be forgiven (more on that later). Problem 3: All sin is willful. As human beings, we have free will. You're not being controlled. "The devil made me do it" will hold up in exactly 0 courts. 1 Corinthians 10:13 even tells us that God will not allow us to be tempted beyond our abilities, and will always provide us a way out of temptation. So what does that tell us about the times we don't bear the temptation? It says we willfully sinned. All sin is willful, and so even if you draw a distinction between sin and willful sin, you must necessarily say that those who sin have lost their salvation, but in that case, we never keep our salvation for long.
We see, then, that losing salvation by works would be as impossible as gaining it by works, and that if we could lose it by works, the cross would be nothing more than just a nice gesture, because no one is good enough to keep salvation. That leads us to the only plausible alternative way to lose salvation: Apostasy.
Now, it's interesting to note that the often cited Hebrews 10:26 actually refers to apostasy. This is especially born out when we compare translations. While some render it "deliberately sin" (HCSB), or "sin willfully" (NKJV), other translations render it "If we deliberately keep on sinning..." (NIV) or "...if we go on sinning deliberately..." (ESV). In other words, it isn't describing the sins of a genuine believer, but the unchanged heart of an apostate.
The obvious example would be Judas Iscariot. Famous for having betrayed the Lord for a mere 30 pieces of silver, Judas Iscariot was nevertheless a close personal friend of Jesus. He was distinct from the other 11 disciples in only one way: His heart. Every other disciple followed Jesus faithfully, but Judas' motives were far more dubious. We know, for example, that he would steal from the group's money bag (John 12:6). Judas never cared for the poor, nor even for the salvation Christ came to bring. Rather, the whole time, Jesus knew Judas was a devil (John 6:70).
From this, we see that it is entirely possible to not only be a false convert, but to even reach high places within the Church and never receive salvation. Judas, as the apostates described in Hebrews 10:26, never lost his salvation because he never had it to begin with!
But is he not a "special case"? Does it not make sense that even if Judas himself never had faith, that since faith is nevertheless the condition for salvation, then to lose faith is to lose salvation all the same? This is the only time when Conditional Security makes sense, at least from a human point of view. But we do need to consider that humans aren't the only, or even most significant element of salvation.
See, the thing that should be abundantly obvious here is that this all revolves around God. Every step of salvation involves Him. He created the heavens and the earth. He foresaw the Fall. He pre-planned the method of salvation. He chose Abraham as the father of the Jewish nation. He chose Mary out of that nation to bear the Messiah. He walked a sinless life. He ordained the events leading up to, surrounding, and ending His earthly ministry, right up to the crucifixion. He died for your sins. He rose again that He may be the firstborn of many brethren. He ascended into Heaven. He inspired the scriptures. He sent the Gospel across the face of the earth. He created you. He ordained the circumstances by which you would hear the Gospel. At what point do you enter this picture? Simple: Wherever He puts you. The only thing you contribute to your salvation is the sin that makes it necessary, and the faith that makes it active.
Now, here's the thing about that: God is omniscient. He knew every day of your life before He even pronounced the words "let there be light". Do you think apostasy ever takes God by surprise? Did God not foresee the loss of faith? Does God not know the heart of a false convert? Of course He does! And so we must ask the same question as with Judas: Did the apostate lose their salvation, or did they just never have salvation to lose?
Interestingly, John answers this in his first epistle, in which he confidently declares "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us." (1 John 2:19, emphasis mine). So what does that say of the apostate? It tells us that their apostasy proves they never actually were "of us".
See, the thing about the apostate is that even if they genuinely did believe they were Christians, they always held at least one idol above the Lord that prevented them from coming to Him. For Judas, it was money. For others, there could be any number of motives. Let's take a look at the Parable of the Sower:
"...Behold, a sower went forth to sow; And when he sowed, some seeds fell by the way side, and the fowls came and devoured them up: Some fell upon stony places, where they had not much earth: and forthwith they sprung up, because they had no deepness of earth: And when the sun was up, they were scorched; and because they had no root, they withered away. And some fell among thorns; and the thorns sprung up, and choked them: But other fell into good ground, and brought forth fruit, some an hundredfold, some sixtyfold, some thirtyfold. Who hath ears to hear, let him hear.
(...)
Hear ye therefore the parable of the sower. When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side. But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it; Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended. He also that received seed among the thorns is he that heareth the word; and the care of this world, and the deceitfulness of riches, choke the word, and he becometh unfruitful. But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty." (Matthew 13:3-9, 18-23)
From this, we see that false conversion, which leads to apostasy, happens for a number of reasons. Perhaps, as I find is the case for most vocal ex-Christians, they did not really know what Christianity was, and so could not really have received it. Perhaps they became Christians not knowing what it entailed, and valuing their earthly lives over the Lord, they were not serious enough about their faith to endure for it. Perhaps, as I said above, they had idols: Wealth, power, pleasurable sins etc. And so they leave for that reason. Note, however, that these are all the bad categories. The good ground always bears fruit. It never shrinks away or dies. It has varying degrees of fruit, but it never dies.
There is good news for the good ground: If you understand the word, you're serious enough about it to endure persecutions, and you're not beholden to any silly idols, you're not going to apostatise. This is because the Holy Spirit is actually preserving your faith for you! 3 times in scripture, we are told that the Holy Spirit has sealed us. 1 Corinthians 1:22 and Ephesians 1:13-14 both tell us that the Spirit seals us as a guarantee of God's promised salvation, and Ephesians 4:30 confirms that this endures until the day of redemption.
Ephesians 4:30 also warns us that it is possible to grieve Him. In this life, we will continue to face temptation, and unfortunately, nothing stops us from giving in. As we have already established, God is too Holy to even observe sin (Habakkuk 1:13), and sin cannot dwell with Him (Psalm 5:4). What, then, are the implications of His very temples, in whom He dwells (1 Corinthians 3:16), rebelling against Him? Evidently, the Spirit will not be too pleased.
This is the major fallacy in the claim that Eternal Security enables sin. The minor fallacy is that if Eternal Security is true, it doesn't matter if it enables sin, any more than the fact belief in gravity enables us to push people to their deaths means gravity is invalid. But the major fallacy is that Eternal Security, because it is true, discourages sin all the more.
See, as Paul says in Romans 6:1-2, we who have died to sin ought no longer to live in it. It is as absurd as a gambler, having had his debt paid by the very casino to whom he owed it, proceeded to double his debt. There is a major difference between "I am sorry I have grieved you" and "I'm sorry that I am due to receive consequences for grieving you". The latter is entirely selfish, whereas the former recognises the dignity of the one being offended. In this case, God.
A simple question to ask advocates of Conditional Security is "if I can convince you of Eternal Security, will you become more licentious?" If the answer is no, then they already agree with you. We shouldn't need God to threaten us into obedience, we should obey because He, Himself, is Holy, and for this cause, He calls upon us to be holy too (Leviticus 11:44; 1 Peter 1:16). If, by contrast, they confess that fear alone motivates them to obey, their love for God is incomplete, and needs to be refined (1 John 4:18). They may even ironically be in the same boat as one of the bad grounds upon which the seed of the Gospel has landed, as the only reason they are not chasing their idols (i.e. being choked by the thorns) is that they don't particularly want to be burned.
All of the above leads us to the conclusion that salvation cannot be lost, which should be great cause for joy, because if we could lose it, Hebrews 6:4-6 actually tells us we couldn't get it back. Here, we are told that it is impossible to renew those who fall away to repentance. In other words, if you don't believe "once saved, always saved", the only alternative left is "once unsaved, always unsaved". Therefore, we can rest safe in the knowledge that salvation cannot be lost. Our salvation is secure in the God who secures it! We can be confident that we are not of those who draw back, but of those who believe, unto the saving of the soul(Hebrews 10:38-39). This is because we have received this promise from the God who declares the end from the beginning (Isaiah 46:10), and speaks of things which are not as if they were (Romans 4:17). Before Isaac was born, God told Abraham "...a father of many nations have I made thee." (Genesis 17:5), and before we enter the Kingdom, He tells us "ye may know that ye have eternal life, and that ye may believe on the name of the Son of God." (1 John 5:13, emphasis mine).
Therefore, my brethren, rejoice, and sin no more, for your salvation is as secure as the God who gave it to you! It does not depend on you, but on Him. You cannot lose your salvation because it was never yours to lose. Therefore, as Paul said to the Philippians, "...work out your own salvation with fear and trembling." (Philippians 2:12).