I was recently reminded of this when reading the Belgic Confession of Faith articles 22-25. In these great paragraphs, the Confession says that all things “requisite to our salvation” are found in Christ and that those who “possess Jesus Christ through faith have complete salvation in him.” It also says that the Christian’s good works “are of no account towards our justification.” In fact, it says, “though we do good works, we do not found our salvation upon them.” Here’s how John Calvin similarly discussed this in his commentary on Hebrews 6:10 (God is not unjust; he will not forget your work and the love you have shown him [NIV]).
But here a difficulty arises, because he seems to say that God is bound by the services of men: “I am persuaded,” he says, “as to your salvation, because God cannot forget your works.” He seems thus to build salvation on works, and to make God a debtor to them. And the sophists, who oppose the merits of works to the grace of God, make much of this sentence, “God is not unrighteous.” For they hence conclude that it would be unjust for him not to render for works the reward of eternal salvation. To this I briefly reply, that the Apostle does not here speak avowedly of the cause of our salvation, and that therefore no opinion can be formed from this passage as to the merits of works, nor can it be hence determined what is due to works. The Scripture shews everywhere that there is no other fountain of salvation but the gratuitous mercy of God: and that God everywhere promises reward to works, this depends on that gratuitous promise, by which he adopts us as his children, and reconciles us to himself by not imputing our sins. Reward then is reserved for works, not through merit, but through the free bounty of God alone; and yet even this free reward of works does not take place, except we be first received into favour through the kind mediation of Christ.
We hence conclude, that God does not pay us a debt, but performs what he has of himself freely promised, and thus performs it, inasmuch as he pardons us and our works; nay, he looks not so much on our works as on his own grace in our works. It is on this account that he forgets not our works, because he recognises himself and the work of his Spirit in them. And this is to be righteous, as the Apostle says, for he cannot deny himself. This passage, then, corresponds with that saying of Paul, “He who has begun in you a good work will perfect it.” (Phil. 1:6.) For what can God find in us to induce him to love us, except what he has first conferred on us? In short, the sophists are mistaken in imagining a mutual relation between God’s righteousness and the merits of our works, since God on the contrary so regards himself and his own gifts, that he carries on to the end what of his own good-will he has begun in us, without any inducement from anything we do; nay, God is righteous in recompensing works, because he is true and faithful: and he has made himself a debtor to us, not by receiving anything from us; but as Augustine says, by freely promising all things.