12 But, O LORD of hosts, that triest the righteous, and seest the reins and the heart, let me see thy vengeance on them: for unto thee have I opened my cause.
For God does not hold out against his children when they beg his pity. And for you he will pray purely, held in high honor as an angel of God, and grieved not by you but for you. This is sincere repentance. “God is not mocked,” nor does he give heed to vain words. For he alone searches the innermost recesses of the heart, and hears those that are in the fire, and listens to those who supplicate in the whale’s belly and is near to all who believe, and far from the ungodly if they do not repent.
The Lord alone is able to certify justice, in the same way that he alone sees the interior of a person’s heart. Hence, Jesus knows the thoughts of people not as an acquired skill, as some allege, but because he is God by nature. Such is what the psalmist sings: “No living creature will be justified in your presence.” If none of those living in virtue are justified, how much more true will this be of those who are dead from sin! Even though the just person knows himself to have God as a defender, the impatience of human fragility desires to see right now what it knows to be coming. Jeremiah also entrusted his cause to God, to the one who said elsewhere, “Vengeance is mine. I will repay, says the Lord.” But the conscience is happy whose cause is entrusted to the Lord, as the apostle said: “Anything that is visible is light.”
Those who are poor in spirit and who have no riches to give them consolation in this world, concerning whom Paul said, “only let us be mindful of the poor,” worshiped the Lord in spirit and gave thanks for being delivered from the hand of the wicked when they obtained vengeance from the Lord. Yet, none of this is due to our merit but to the grace of him who liberated the poor, of him who possesses none of the riches of corrupting pride but has the humility of the poor who are free.
(Verse 12.) And you, Lord, the tester of the righteous (or the one who tests the righteous), who sees the reins and the heart, may I please see your vengeance from them, for I have revealed my cause to you (Luke 6). The Lord alone is the one who knows how to test righteousness, just as he alone is the one who can see the inner thoughts of the heart. Thus, Jesus, knowing the thoughts of men, is not from advancement, as some may think, but by nature is God. Something similar is also expressed in the Psalms: No one living shall be justified in your sight (Psalm 143:2). If one who lives by virtues will not be justified, how much more so one who is dead because of sins? And even though he knows he has God as his advocate, still in the impatience of human frailty, he desires to see what he knows will come in the future, even now. And he to whom he revealed his cause, who says in another place: Vengeance is mine, and I will repay, says the Lord (Deut. 32:35). But happy is the conscience for whom the cause is revealed to the Lord (Heb. 10:30), as the Apostle says: Everything that is revealed is light (Eph. 5:13).
[AD 215] Clement of Alexandria on Jeremiah 20:12