Calvin:

O if my people had hearkened to me! By the honorable designation which God gives to the people of Israel, He exposes the more effectually their shameful and disgraceful conduct. Their wickedness was doubly aggravated, as will appear from the consideration, that although God called them to be his people, they differed nothing from those who were the greatest strangers to him. Thus he complains by the Prophet Isaiah, “The ox knoweth his owner, and the ass his master’s crib: but Israel doth not know, my people doth not consider.” (Isaiah 1:3) The Hebrew particle lu, which I have rendered O if! is not to be understood as expressing a condition, but a wish; and therefore God, I have no doubt, like a man weeping and lamenting, cries out, O the wretchedness of this people in wilfully refusing to have their best interests carefully provided for! He assumes the character of a father, and observing, after having tried every possible means for the recovery of his children, that their condition is utterly hopeless, he uses the language of one saddened, as it were, with sighing and groaning; not that he is subject to human passions, but because he cannot otherwise express the greatness of the love which he bears towards us.

The Prophet seems to have borrowed this passage from the song of Moses in Deuteronomy 32:29, where the obstinacy of the people is bewailed in almost the same words: “Oh that they were wise, that they understood this, that they would consider their latter end!” He means tacitly to upbraid the Jews, and to impress upon their minds the truth, that their own perverseness was the only cause which prevented them from enjoying a state of great outward prosperity.

If it is objected, that God in vain and without ground utters this complaint, since it was in his power to bend the stiff necks of the people, and that, when he was not pleased to do this, he had no reason to compare himself to a man deeply grieved; I answer, that he very properly makes use of this style of speaking on our account, that we may seek for the procuring cause of our misery nowhere but in ourselves. We must here beware of mingling together things which are totally different as widely different from each other as heaven is distant from the earth. God, in coming down to us by his word, and addressing his invitations to all men without exception, disappoints nobody. All who sincerely come to him are received, and find from actual experience that they were not called in vain. At the same time, we are to trace to the fountain of the secret electing purpose of God this difference, that the word enters into the heart of some, while others only hear the sound of it. And yet there is no inconsistency in his complaining, as it were, with tears, of our folly when we do not obey him.

In the invitations which he addresses to us by the external word, he shows himself to be a father; and why may he not also be understood as still representing himself under the image of a father in using this form of complaint? In Ezekiel 18:32, he declares with the strictest regard to truth, “I have no pleasure in the death of him that dieth,” provided in the interpretation of the passage we candidly and dispassionately take into view the whole scope of it. God has no pleasure in the death of a sinner: How? Because he would have all men turned to himself. But it is abundantly evident, that men by their own free-will cannot turn to God, until he first change their stony hearts into hearts of flesh: yea, this renovation, as Augustine judiciously observes, is a work surpassing that of the creation itself.

Now what hinders God from bending and framing the hearts of all men equally in submission to him? Here modesty and sobriety must be observed, that instead of presuming to intrude into his incomprehensible decrees, we may rest contented with the revelation which he has made of his will in his word. There is the justest ground for saying that he wills the salvation of those to whom that language is addressed, (Isaiah 21:12,) “Come unto me, and be ye converted.” In the second part of the verse before us, we have defined what it is to hear God. To assent to what he speaks would not be enough; for hypocrites will grant at once that whatever proceeds from his mouth is true, and will affect to listen just as if an ass should bend its ears. But the clause is intended to teach us that we can only be said to hear God, when we submit ourselves to his authority. Calvin, Commentary, Psalm 81:13.