(Hebrews 6:4–6) For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
Notice that it says, "it is impossible to renew them again", not, "it is impossible for them to be renewed". The phrase "renew them again" refers to Christian teachers and preachers teaching Christian truth in the hopes that their hearers will advance in the faith (and stop being babies). But those who "fall away" are no longer in the audience. This passage does not say that it is impossible for them to be saved nor does it say that it is impossible for them to again become Christians (if they ever were Christians at all).
This passage is typically interpreted to mean that if a Christian renounces Christ and Christianity, they can never again return to the faith. But a careful reading shows that this is not what the words actually say (as I highlight in the previous paragraph).
This typical interpretation causes problems because it contradicts various other cherished Christian teachings including:
This passage is not really a problem at all when read carefully. Here is the extended passage with excess phrases and sentences pruned away:
You have need that one teach him again (5:12) for he is a baby (5:13) and dull of hearing (5:11) and cannot discern between good and evil (5:14). But, let us instead go on to perfection (6:1) and this we would do, but can't (6:3), because it is impossible for one who was a Christian (6:4-5), if he falls away, to renew him again (6:6).
Notice that the one who fell away is a baby who no longer receives the life-giving rain.
(Hebrews 6:7,8) For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God: But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.
It is clear that a person must make progress in their faith in order to finally be redeemed. This contradicts the typical fundamentalist evangelical Protestant view that once you accept Jesus in an emotional moment of belief you can never lose your salvation no matter how much you sin. Unfortunately, this view is false as I discuss elsewhere (here and here).
Back to the passage at hand. Why is it impossible for this person who apostatized to again become a Christian (ignoring whether they lost their salvation or never had it in the first place)? The key is in noticing what it is that is impossible. It comes down to this:
It is impossible to renew him again (if he falls away, because he no longer receives proper gospel teaching since he doesn't attend sermons anymore).
If a person who was a baby rejects the elementary teachings and won't listen to sermons anymore, it is impossible to renew him again. The reason it is impossible is because they are not available to renew. In order to renew someone you have to be able to speak with them, but the person in question is shunning interaction with Christian teachers.
Notice what is missing in all this. This passage makes no reference to whether or not the person is still saved or not. Here's the flow of ideas:
Here are the characteristics (6:4-5) of the person who later apostatized:
Notice that none of these imply that the person was saved — maybe they were, maybe they weren't.
People usually interpret the sequence of events of this person as follows:
I find the following sequence more satisfying: