Some people "once had"... God's light. They experienced the heavenly gift and were "partakers".... in the Holy Spirit. They experienced the goodness of God's word and the powers of the world to come. And then..... they abandoned their faith! (the true faith)
It is impossible to bring them back to "REPENT AGAIN", because they are again crucifying the Son of God and exposing Him to public shame.

THEREFORE, they cannot be led a "second time" to God. (Hebrews 6:4-6)

*There are some people who will abandon their faith in God and thus forfeit the salvation they once had.

"But the one who endures to the end, he will be saved.' - Jesus (Matthew 24:13)