One of the ugliest, least popular passages in the Bible talks about the immoral woman. It tells us in Proverbs 2:
18 For her house leads down to death, And her paths to the dead;
19 None who go to her return, Nor do they regain the paths of life ---
A Christian who gets involved in immorality can be forgiven, cleansed, and used by God. But this passage teaches that he will never fully recover. There is nothing in Scripture to suggest that John Mark had been involved in immorality, but there is a principle here: certain sins can leave results that repentance does not remove.