Hebrews 6:4-8

how do we know if someone falls in those this?
For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. 7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. 8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.

when also there is romans 11:23 which says those who dont believe can be grafted back in if they are cut off

and the prodigal as well which the person alieve and became dead and then became alieve again

What the author of Hebrews is discussing in context is someone who has been a Christian and who goes back to Judaism. This would be a form of apostasy.

They would have renounced the grace of Jesus, and would also no more have the benefits of Judaism.

It doesn’t mean that they would be forever unforgiven, though. But if they remained in their apostasy, they would have no means for forgiveness.

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