Hebrews 6:4
It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, — Warning 3 is the most severe of Hebrews' cautionary passages: apostasy after genuine spiritual experience is impossible to reverse—not impossible in the sense that God cannot forgive, but impossible in the sense that no second repentance is available. The markers of genuine faith describe real spiritual experience.