Two of the biggest questions in Christianity are can we lose our salvation, and if so, how? The answer to the first is a very clear no. Salvation is a gift from God. It is not something that can be lost, because once it has been given, it is also maintained. If you could lose your salvation, it is guaranteed you would, but as it stands, the Bible makes it quite clear that all three persons of the Trinity work together to ensure that once you have salvation, nothing you do can result in the loss thereof.
This brings us to the second question, which actually helps us solidify our answer to the first. If we could lose our salvation, what sin/s would cause that? The answer to that is almost always ambiguous. We don't know when salvation is lost and needs to be re-acquired (if it even can be). This is in direct conflict with 1 John 5:13. If some ambiguous sin could cause us to lose salvation, we could not know that we have eternal life. We would either not know if we have eternal life, or we would only know that we have the possibility of eternal life. As it happens, the verse says we know we have eternal life.
There is a single exception to the ambiguity: Apostasy, supposedly, results in the loss of salvation. This is not ambiguous, however it is also answered, and by none other than this same John. In 1 John 2:19, John explains that not only were apostates never truly of the faith, but also that the very reason they left the faith is so that it may be manifest that they were never of it. A true Christian cannot apostatise. Why? Because God has sealed us with the Holy Spirit as a guarantee of eternal life. We are sealed for the day of redemption by the Holy Spirit, guaranteeing our eternal life.
This is where proponents of Conditional Security start saying that if we are saved without potential to lose salvation, we may as well just sin to our heart's content. There are two very important comebacks for this. The first is that it is very telling. Of course, no man is without sin, but if you're going to confess to being so deep in sin that the only reason you avoid it is because you believe you'll go to Hell if you do sin, what you're actually saying is that the Holy Spirit has not yet been effective enough in your life. The Christian man will sin, but it will be a war with the flesh, as we will naturally hunger and thirst for righteousness.
Which leads us to the second point. In the same verse where Paul tells us we are sealed for the day of redemption by the Holy Spirit, we are given a very clear command: Do not grieve Him. This, first of all, reinforces the point. If the Holy Spirit who seals us can be grieved, that means it is entirely possible to sin, yet still be sealed. Otherwise, it wouldn't be a seal, it would be a temporary residence. It would say something like "do not forfeit the Holy Spirit, who hopes to carry you through to the day of redemption." But no, this concept of being sealed as a guarantee of our inheritance is repeated throughout the scriptures.
And so we see that salvation just cannot be lost. Once given, God does not take it back. If one apostatises, they don't lose their salvation, they prove they never had it (though it should be noted that both sides of the issue believe in the same result for the apostate, making this part of the discussion purely academic). If salvation could be lost, God would not have been so ambiguous about how it is done. Furthermore, this ambiguity would eliminate the confidence the scriptures seek to instil in us. We could never know if we had eternal life if it was possible to lose somehow, but we didn't know how. Especially given that, one way or another, we all sin. Therefore, we can safely conclude that it is 100% impossible to lose salvation.