In Hebrews 10, the writer points to “the law,” the covenant of Israel, and notes that it never really was the last word. He’s already made that point in chapter 8 with the promise of God in Jeremiah of a new covenant. The problem with “the law,” is that it could not effect change in the lives of people who drew near to God to worship. He explains what he means in verse 2. This perfection is a continually cleansed state, free from guilt for sin.
But the continued offering of the sacrifices showed that nothing had changed in Israel’s life.
When Christ came, however, he offered a superior, once for all sacrifice and we have, by that sacrifice, been cleansed once for all and been made, in God’s eyes, perfect.
“Perfect,” however should not be construed as “sinless.” The writer’s point is that we have been raised to a different status, the same status as that of a risen Jesus (note Jesus was made “perfect” through suffering in 2:10). Given that status, a people for whom the guilt of sin is no longer an impediment to our lives, we are not allowed to press on persistent in sin. The one who does that has no hope, for there was one sacrifice in our behalf. There remains no further sacrifice.
The point of this is that when Christians sin, as they undoubtedly will, God simply forgives them. The idea is to learn from the mistake and do better. The life however that makes no attempt to do better ultimately ends up in a land of no return and will discover what a fearful thing it is to fall into the hands of the living God.