May I add:
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, and have tasted the goodness of the word of God and the powers of the age to come, 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. 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. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
Hebrews 6:4-8
Thus,
it is possible — on the witness of Paul — for men to Believe, and then later to Backslide.
To wit, it is possible — on the witness of Paul — for men to Believe, but to still produce no "fruit" but only "weeds". (Indeed,
the Messiah says specifically this in the
Parable of the Sower, Matt 13:21-22.)
How can I possibly have mis-interpreted the plain meaning of these words? Paul would not speak of impossibilities. Paul would not speak of the moon being made of cheese, for that would serve no conceivable purpose. But rather Paul warns against Believing and Backsliding — it can, and has happened, not the least of which from the days of Aaron and the Golden Calf (Ex 32) through the books of Judges and Kings (Chronicles).
Indeed further, this is exactly why Paul wrote Hebrews to the Jerusalem congregation in c. 62 AD — for, James
haTzaddik ("the Just"), the head pillar of the church, had just been murdered by the High Priest Ananias at Passover. The Jerusalem congregation was gravely demoralized, and Paul wrote to them to urge them back onto the right path.