It is impossible for those who have once been enlightened…
Hebrews 6:4
I don’t think that is what it says, especially in light of and in conjunction with the other evidence already presented (and I’m not pretending to have been exhaustive – there is probably more). The author of Hebrews is presenting an impossible hypothetical – it’s an impossibility if someone does fall away that was truly in God’s family. He is not saying that could actually happen.
So there is still hope for ‘deconverted Christians’, [otherwise not, according to you.] [ETA]