Jeremiah 17:10

The Pharisees, by justifying themselves before people, were placing in people their hope of reward. His rebuke to them had the same bearing as that of the prophet Jeremiah, "Cursed is the one who trusts in humankind." He says next, "But God knows your hearts." This was a reference to the power of that God who declared himself a shining light, "searching the hearts and the seats of emotions." If he turns with hostility toward their pride, "what is exalted among people is an abomination in the sight of God," he sets Isaiah in front of their eyes, "For the LORD of hosts has a day against all that is proud and lofty, against all that is lifted up and high."

(Verse 9, 10.) The heart of all is perverse and inscrutable, who can understand it? I am the Lord, searching the heart and testing the kidneys, who gives to each according to their ways and according to the fruit of their inventions. LXX: The heart is deep above all, and man is, who can know him? and so on. The Hebrew word Enos is written with four letters, Aleph and Nun and Vau and Sin. Therefore, if Enos is read, it means 'man', but if Anus, it is inscrutable or desperate, because no human heart can find it. But Symmachus interpreted this passage as follows: Inscrutable is the heart of all: but who is the man that can find it? Some of ours, indeed, with good intentions, but not according to knowledge, use this passage against the Jews, because He is called Lord and Savior, according to the dispensation of the assumed flesh, and no one can know the mystery of His birth, according to what is written: Who shall declare His generation? (Isaiah 53:8) Unless God alone who searches the depths and renders to each according to his works. However, it is better that we simply accept that no one knows the secrets of our thoughts except God alone. For as it was said above: Cursed is the man who has hope in man. And conversely: Blessed is the man who trusts in the Lord. Hence, in order for us not to think that the judgment of men is certain, it was introduced that the hearts of almost all are perverse, as the Psalmist says: Cleanse me from my hidden faults, and spare your servant from the faults of others (Psal. 19:13): undoubtedly referring to thoughts. And in Genesis: But God, seeing that the wickedness of men was great on the earth, and that every thought and intent of the heart was inclined towards evil at all times (Gen. 6:5). And again: For the sense and thought of the human heart are prone to evil from their youth (Gen. 8:21). […]

But that it belongs to God only to know men's secrets, hear what saith the prophet, "Thou most entirely alone knowest the hearts;" and again, "God trieth the hearts and reins;" and Jeremiah too saith, "The heart is deep above all things, and it is man, and who shall know him?" and, "Man shall look on the face, but God on the heart." And by many things one may see, that to know what is in the mind belongs to God alone.

"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.

God has promised that Christ will be a light and has declared that he himself is a lantern, searching the hearts and seats of affections.