Augustine of Hippo, on Jer 17:10

“I am the Lord who search the heart and prove the reins: who give to every one according to his way, and according to the fruit of his devices.”

“"And Thou shalt direct the righteous, O God, who searcheth the hearts and reins." How then can the righteous be directed but in secret? ...How then is the righteous man directed in so great confusion of pretence, save whilst God searcheth the hearts and reins; seeing all men's thoughts, which are meant by the word heart; and their delights, which are understood by the word reins? ...God then, searching our heart, and perceiving that it is there where our treasure is, that is, in heaven; searching also the reins, and perceiving that we do not assent to flesh and blood, but delight ourselves in the Lord, directs the righteous man in his inward conscience before Him, where no man seeth, but He alone who perceiveth what each man thinketh, and what delighteth each. For delight is the end of care; because to this end does each man strive by care and thought, that he may attain to his delight. He therefore seeth our cares, who searcheth the heart. He seeth too the ends of cares, that is delights, who narrowly searcheth the reins; that when He shall find that our cares incline neither to the lust of the flesh, nor to the lust of the eyes, nor to the pride of life, all which pass away as a shadow, but that they are raised upward to the joys of things eternal, which are spoilt by no change, He may direct the righteous, even He, the God who searcheth the hearts and reins. For our works, which we do in deeds and words, may be known unto men; but with what mind they are done, and to what end we would attain by means of them, He alone knoweth, the God who searcheth the hearts and reins.”

Imported from an open dataset — not yet checked against the printed edition.