Calvin on Jer.9:13,14 :"And they have walked after the hardness, or obstinacy, or imaginations, of their own heart See Note on Jeremiah 3:17, 18. He opposes the imaginations, or hardness of the heart, to the voice of God, as we find in other places, where contrary things are stilted, that is, what men’s minds devise, and what God shews by his word to be right; for there is no less contrariety between the rule of right living and the imaginations of men, than there is between fire and water. Let us therefore know, that our life cannot be rightly formed except we renounce our own imaginations, and simply obey the voice of God: for as soon as we yield the least to our own imaginations, we necessarily turn aside from the right way, which God has made known to us in his word. This contrast, then, between the law of God and the imaginations or the obduracy of men ought to be carefully noticed.

He then more clearly explains how they had sinned, and after Baalim. It is supposed that the Israelites made a difference between this word and God: they allowed but one God, but introduced Baalim, or inferior gods, and worshipped them. They tried to evade the charge of idolatry, by alleging that Baalim were mediators. But no excuse of this kind was admitted, as God everywhere imputed idolatry to them. Notwithstanding this example, and the distinct declaration of Scripture, that there is but one God and one Mediator, (1 Corinthians 8:5, 6; 1 Timothy 2:5,) the error, the awful error of praying to saints, etc., as mediators, has prevailed in the Christian Church! — Ed. The Prophet here adds nothing new; but by specifying one thing he shews how the Jews followed their own imaginations, by giving themselves up to profane superstitions. What indeed must happen to men, when they forsake God, and allow themselves to follow their own thoughts? what but error and superstition, yea, the abyss of all errors? In short, the Prophet in this clause intended to cut off every occasion for subterfuges; for the Jews, like hypocrites, who sophistically deal with God, might have made this evasion, and said, “Why dost thou object to us our imaginations? what are these imaginations?” Baalim, he says, “Ye have devised idols far yourselves in addition to the only true God; it is hence quite evident, that having forsaken God’s word, ye have followed your own imaginations.”

"Let us therefore know, that our life cannot be rightly formed except we renounce our own imaginations, and simply obey the voice of God: for as soon as we yield the least to our own imaginations, we necessarily turn aside from the right way, which God has made known to us in his word. This contrast, then, between the law of God and the imaginations or the obduracy of men ought to be carefully noticed."Calvin, Jer.9:13,14, "Who can know the mind of the Lord?"