"4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 [c]if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame." - Hebrews 6:4-6
The "falling away mentioned is not just backsliding or temporarily going back into the world for a while and then being retrieved. For individuals who have this experience, the flesh is destroyed so that "the spirit may be saved in the day of the Lord Jesus (1 Cor. 5:5). Hence, they are retrieved and do not fall away" in the sense of the context here in chapter 6.
Proverbs 24:16, which reads, "A just righteous man falleth seven times, and riseth up again," refers to a different kind of falling, whereas verses 4-6 refers to a permanent falling away from the Lord of "falling." 6 unto extinction, or Second Death. For example, although it is dangerous for brethren to stop attending meetings, that action in itself does not indicate a Second Death condition in which it is impossible to return to the Lord, that is, a falling away from the brethren is not necessarily synonymous with a falling away from the Lord, for He may take certain other circumstances into consideration and be merciful in His judgment. Paul was speaking of a class whom it is impossihle to renew unto repentance because they have committed the sin unto Second Death.