This verse is repeated almost verbatim from Jeremiah 11:20.

But, O Lord, that triest the righteous, and seest the reins and the heart,.... That tries the cause of the righteous, and vindicates them, rights their wrongs, and does them justice; being the omniscient God, the seer and searcher of the hearts and reins; who knows the uprightness of their hearts, as well as their ways, and the sincerity of their affections: or that tries the faith and patience of the righteous, their constancy and integrity; and upon trial finds that their affections are real, and their souls sincere;

let me see thy vengeance on them; his enemies and persecutors; he does not seek vengeance himself, but desires it of the Lord; he does not ask to see his vengeance, but the Lord's vengeance on them, what he thought was just and proper to inflict on them; he knew that vengeance belonged to the Lord, and therefore left it with him, and prayed for it from him. The Targum is,

"let me see the vengeance of thy judgments on them;''

for unto thee have I opened my cause; or "revealed", or "made it manifest" (k); this he did in prayer, at this time, when he laid before the Lord his whole case, and appealed and applied to him for justice, who judgeth righteous persons, and judgeth righteously.

(k) "revelavi causam meam", Junius & Tremellius, Piscator. So Schmidt.

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.

12. triest the righteous-in latent contrast to the hasty judgments of men (Jer 11:20; 17:10).

opened-that is, committed (compare 2Ki 19:14; Ps 35:1).

20:7-13 The prophet complains of the insult and injury he experienced. But ver. 7 may be read, Thou hast persuaded me, and I was persuaded. Thou wast stronger than I; and didst overpower me by the influence of thy Spirit upon me. So long as we see ourselves in the way of God, and of duty, it is weakness and folly, when we meet with difficulties and discouragements, to wish we had never set out in it. The prophet found the grace of God mighty in him to keep him to his business, notwithstanding the temptation he was in to throw it up. Whatever injuries are done to us, we must leave them to that God to whom vengeance belongs, and who has said, I will repay. So full was he of the comfort of God's presence, the Divine protection he was under, and the Divine promise he had to depend upon, that he stirred up himself and others to give God the glory. Let the people of God open their cause before Him, and he will enable them to see deliverance.