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 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, NKJV)
Verse 6 gives the reason for the impossibility of renewal to repentance if Christians fall away. When fallen Christians continue to practice their sins, they forfeit the power to renew their faith. It is impossible to be renewed spiritually while continuing to sin. They are crucifying the Son of God by their willful transgressions, openly shaming Him by their return to and continuance in sin (Heb. 10:26-31). Indulging in sin instead of resisting it hardens hearts that were once enlightened, enlivened, and edified by God’s word and its promised hope. Therefore, we are warned not to let sin have a place in our hearts and lives (Heb. 3:12-13). Yes, Christians can fall away and be lost (2 Pet. 2:20-22; Gal. 5:4). And yes, fallen saints can be restored to Christ, but only by strictly putting away the sins that have prevented their repentant return to the Lord. Willful sin must cease for divine mercy to take its place (Lk. 15:17-24).