This quote is from John Calvin’s commentary on Hebrews chapter 6, discussing the “falling away” issue.
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.2
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.
This is an outstanding discussion of what it means for someone to fall away from the grace of God. And clearly, anyone who thinks that denying God’s grace or falling away from that grace is inconsistent with “Calvinism” would properly be accused of being more “Calvinist” than Calvin himself.
Note that last paragraph in particular- the doctrine of eternal security must be properly understood or can very easily become a cloak for antinomianism and pride. God warns us constantly of the danger of self-reliance. Calvin warns us against “security and foolish confidence.” God’s elect will never fall away from the grace of Jesus Christ, but God works through means, and one of His means are warnings such as we see in Hebrews 6. The elect of God will hear these warnings and listen. The elect of God will never say, “I’m elect, and therefore that warning doesn’t apply to me.”
And once we understand that truth, we will never again feel any conflict between the doctrine of the perseverance of the saints, and the warnings that Scripture constantly gives us against falling away.