Why would God require of man that which he can not do of his own will?
“They set chief stock by God’s precepts. These they consider to be so accommodated to our capacities that we are of necessity able to fulfill all their demonstrable requirements. Consequently, they run through the individual precepts, and from them take the measure of our strength. Either God is mocking us (they say) when he enjoins holiness, piety, obedience, chastity, love, gentleness; when he forbids uncleanness, idolatry, immodesty, anger, robbery, pride, and the life; or he requires only what is within our power.
Now we can divide into three classes almost all the precepts that they heap up. Some require man first to turn toward God; others simply speak of observing the law; others bid man to persevere in God’s grace once it has been received. We shall discuss them all in general, then we shall get down to the three classes themselves.
A long time ago it became the common practice to measure man’s capacities by the precepts of God’s law, and this has some pretense of truth. But it arose out of the crassest ignorance of the law. For, those who deem it a terrible crime to say that it is impossible to observe the law press upon us as what is evidently their strongest reason that otherwise the law was given without purpose. Indeed, they speak as if Paul had nowhere spoken of the law. What then, I ask, do these assertions mean: “The law was put forward because of transgressions” [Gal. 3:19]; “Through the law comes knowledge of sin” [Rom. 3:20]; the law endangers sin [cf. Rom. 7:7-8]; “Law slipped in to increase the trespass” [Rom. 5:20]? Was the law to be limited to our powers so as not to be given in vain? Rather, it was put far above us, to show clearly our own weakness! Surely, according to Paul’s definition of the law, its purpose and fulfillment is love [cf. 1 Tim. 1:5]. And yet when Paul prays for the hearts of the Thessalonians to abound with it [1 Thess. 3:12] he fully admits that the law sounds in our ears without effect unless God inspires in our hearts the whole sum of the law [cf. Matt. 22:37-40].
-Calvin, Calvin’s Institutes, Book II, Ch. V.6