Many people stumble on a verse that is unfortunately taken out of context: Heb 10:26 "For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins..."
The point of the book of Hebrews was to show the superiority of Jesus and His one sacrifice through which sin was taken away and through which God remembers our sins no more, over the Old Covenant system in which sins were never taken away and in fact were only remembered! Hebrews 10:26 isn't saying that Jesus' sacrifice doesn't apply to people who sin willfully. That would contradict the whole point the writer was making! Rather, as we look at the verse in context, we see that the point is that Jesus' one sacrifice does indeed apply to people who sin, and that there is no other sacrifice that they can turn to.