GENERAL STATEMENTS IN THE BIBLE
What is the point of the story of the prodigal son, and how does it relate to Hebrews 6:6, if it can relate at all?
I would say that we here have a very fine example of "general statements"
I think we need to put other words where we have in verse 4 "it is" in italics. NOT in the original Greek.
I think we need to understand this passage as "For it is sometimes and often impossible....." etc.
The prodigal son shows it is not always impossible to come back to God in this lifetime, when someone has left God.
As long as there is physical life in a person who once knew the truth and walked in it, then walks away, it is not a closed door that can not be opened again. Yes for many who once walked in the truth of God, and have walked away, most of them in this life may not come back, BUT a few may. Paul I'm sure knew this, so putting scripture with scripture, Paul was using a general statement; hence better understood as "For [it is sometimes and often] impossible for those....."
As Peter [2 Peter 3:9] said, God is not wanting any to perish but that all may come to repentance. That coming to repentance can be at any time in this physical life.
Ah yes this shows the very important key to reading ALL the Bible, for the Bible is full of "general statements" AS like in the book of PROVERBS.....loaded with general statements.