Having been in the church a while, I’ve seen and heard things. This verse has always troubled me:
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.
Hebrews 6:4-6 English Standard Version (ESV) emphasis mine
I had once presented the gospel to a fellow who said to me that he knew he was not redeemable because of these verses. Ignorant as I was at the time, I tried to convince him that there was always hope while he yet lived. He, a non-believer, quoted these verses to me, thanked me for my concern, and wandered away. I never saw him again.
John Calvin comments:
Let us then know, that the Gospel cannot be otherwise rightly known than by the illumination of the Spirit, and that being thus drawn away from the world, we are raised up to heaven, and that knowing the goodness of God we rely on his word.
But here arises a new question, how can it be that he who has once made such a progress should afterwards fall away? For God, it may be said, calls none effectually but the elect, and Paul testifies that they are really his sons who are led by his Spirit, (Romans 8:14) and he teaches us, that it is a sure pledge of adoption when Christ makes us partakers of his Spirit. The elect are also beyond the danger of finally falling away; for the Father who gave them to be preserved by Christ his Son is greater than all, and Christ promises to watch over them all so that none may perish.
To all this I answer, That God indeed favors none but the elect alone with the Spirit of regeneration, and that by this they are distinguished from the reprobate; for they are renewed after his image and receive the earnest of the Spirit in hope of the future inheritance, and by the same Spirit the Gospel is sealed in their hearts.
But I cannot admit that all this is any reason why he should not grant the reprobate also some taste of his grace, why he should not irradiate their minds with some sparks of his light, why he should not give them some perception of his goodness, and in some sort engrave his word on their hearts.
Otherwise, where would be the temporal faith mentioned by Mark 4:17? There is therefore some knowledge even in the reprobate, which afterwards vanishes away, either because it did not strike roots sufficiently deep, or because it withers, being choked up.
And by this bridle the Lord keeps us in fear and humility; and we certainly see how prone human nature is otherwise to security and foolish confidence. At the same time our solicitude ought to be such as not to disturb the peace of conscience. For the Lord strengthens faith in us, while he subdues our flesh: and hence he would have faith to remain and rest tranquilly as in a safe haven; but he exercises the flesh with various conflicts, that it may not grow wanton through idleness.
So the bitter admonition in these verses serves us well: to keep us striving for holiness, humility, and obedience to the Lord Jesus Christ in all we think, say, and do.