I think this has been covered in separate threads, but it is good for a review.
Heb 6:1-6 has to be interpreted and understood in the context in which it was written.
The context of the passage which actually begins in chapter 5 verse 12 is not salvation. It is doctrine. He is talking about viable doctrine, babyhood "foundational" doctrine, and the possibility of going on to a mature "perfect" doctrine.
Eventually, as he gets to verses 3-8 of chapter 6, he is showing how unworkable the Jewish Christian's doctrine is going to be if they attempt to alter the foundational teaching about repentance from dead works. The Jewish Christians were wanting to take the priesthood that was instituted in the NT and merge it with the priesthood of Aaron that existed in the OT. Doing so would totally mess up the NT concept of Jesus being a sacrifice who died once and for all. The OT sacrifice was offered yearly, so if the OT and NT understanding of the sacrifice for sins were merged, then we would be sacrificing Jesus again and again, something that cannot be done.
Look at the verses in question:
Hebrews 6:1-3 GW
1 With this in mind, we should stop going over the elementary truths about Christ and move on to topics for more mature people. We shouldn't repeat the basics about turning away from the useless things we did and the basics about faith in God.
2 We shouldn't repeat the basic teachings about such things as baptisms, setting people apart for holy tasks, dead people coming back to life, and eternal judgment.
3 If God permits, we will do this.
Notice that nowhere is he talking about salvation. He is talking about going on to a mature and complete doctrinal understanding. He goes on to point out that they will not be able to go on to this complete and mature doctrinal understanding if they do not first stand upon the foundational truths... one of which is "repentance from dead works." It is here that they are stumbling. Dead works are not sins... they are the OT rituals and sacrifices that did not actually wash away sins or bring anyone closer to God. Paul is pointing out that if they take the NT teaching about repentance from dead works and merge it with the OT teachings about yearly sacrificing for sins, then they will have a mongrel doctrine that requires that Jesus go to the cross again and again and again... on a yearly basis.
Hebrews 6:4-6 WNT
4 For it is impossible, in the case of those who have once for all been enlightened, and have tasted the sweetness of the heavenly gift, and have been made partakers of the Holy Spirit,
5 and have realized how good the word of God is and how mighty are the powers of the coming Age, and then fell away--
6 it is impossible, I say, to keep bringing them back to a new repentance, for, to their own undoing, they are repeatedly crucifying the Son of God afresh and exposing Him to open shame.
Paul is showing how unworkable this hybrid doctrine would be. "It (the hybrid doctrine they are creating) is impossible" because it would require the believer to go through repentance from dead works again and again just like they had to in the OT. This hybrid doctrine would have Jesus repeatedly going to the cross every year just like the OT law required. It is the hybrid doctrine that is not viable, unworkable, and that would expose Jesus to open shame ever year as He was sacrifice again and again and again.
