Jeremiah 17:9

The heart is deceitful above all things, and desperately wicked: who can know it?
Read Chapter 17

Ambrose of Milan

AD 397
The apostle says, “For God, sending his own Son in the likeness of sinful flesh, and for sin condemned sin in the flesh, that the righteousness of the law might be fulfilled in us.” He does not say “in the likeness of flesh,” for Christ took on himself the reality, not the likeness, of flesh. Nor does he say in the likeness of sin, for he did not sin but was made sin for us. Yet he came “in the likeness of sinful flesh.” That is, he took on him the likeness of sinful flesh, the likeness, because it is written, “He is man, and who shall know him?” He was man in the flesh, according to his human nature that he might be recognized, but in power he was above humanity, that he might not be recognized, so he has our flesh but has not the failings of this flesh. - "Concerning Repentance 1.3.12"

Augustine of Hippo

AD 430
The inquirer, then, might say that the prophet says only that Christ is God, without any reference to his human nature. Yet, in our apostolic doctrine, Christ is not only God in whom we may safely trust but also the mediator between God and humankind—the man Jesus. The prophet explains this in the words in which he seems to check himself, and to supply the omission: “The heart,” he says “is inscrutable above all things, and he is man, and who shall know him?” He is man in order that, in the form of a servant, he might heal the hard in heart and that they might acknowledge as God him who became man for their sakes, that their trust might be not in humankind, but in God—man. - "Reply to Faustus the Manichaean 13.8"

Augustine of Hippo

AD 430
His manhood was much more plainly and readily recognized by strangers, who, indeed, were not wrong in believing him to be man, but they did not understand his being God as well as man. Hence Jeremiah says, “He is man, and who shall know him?” He is a man, for it is made manifest that he is a brother. And who shall know him? For it is concealed that he is a husband [to the church]. This must suffice as a defense of our father Abraham against Faustus’s imprudence and ignorance and malice. - "Reply to Faustus the Manichaean 22.40"
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Epiphanius of Cyprus

AD 403
How can they declare the Savior a mere man, conceived of a man’s seed? How will he “not be known,” as Jeremiah says of him, “he is man, but who shall know him?” For the prophet, describing him, said, “Who shall know him?” But if he meant a mere man, surely his father would know him, and his mother, his relatives and neighbors, those who lived with him and his fellow townspeople. But that which came to birth is born of Mary, while the divine Word came from above. He was truly begotten not in time and without a beginning, not of a man’s seed but of the Father on high. But in the last days he consented to enter a virgin’s womb and fashion his flesh from her, patterned after himself. This is why Jeremiah says, “And he is man, but who shall know him?” He came from above as God, the only-begotten, divine Word. - "Panarion 30.20.5–7"

George Leo Haydock

AD 1849
Perverse. Septuagint, "deep. " Unsearchable. Septuagint, "man, who shall know him? "(Haydock) God alone can search the heart by his own power. He enables saints to do it by the light of glory, or of prophecy; as Eliseus and St. Peter knew secret transactions. (Worthington)
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Irenaeus of Lyons

AD 202
For this reason it is said, “Who shall declare his generation?” since “he is man, and who shall recognize him?” But one to whom the Father who is in heaven has revealed him knows him, so that he understands that he who “was not born either by the will of the flesh or by the will of man” is the Son of man, that is, Christ, the Son of the living God. For I have shown from the Scriptures that no one of the children of Adam is as to everything, and absolutely, called God or named Lord. But that he is himself in his own right, beyond all people who ever lived, God, and Lord, and King eternal and the incarnate Word, proclaimed by all the prophets, the apostles and by the Spirit, may be seen by all who have attained to even a small portion of the truth. Now, the Scriptures would not have testified these things of him, if, like others, he had been a mere man. But that he had, beyond all others, in himself that pre-eminent birth that is from the most high Father, and also experienced that pre-e...


AD 420
Symmacus translates this passage thus: “The heart of everyone is inscrutable. Yet, who is the man who can find it?” It is customary by our own good wish, therefore, but not according to objective knowledge, to use this passage to argue against the Jews that the man in question is the Lord and Savior, according to the dispensation of the assumed flesh, and that no one shall be able to know the mystery of his nativity, according to what is written: “Who will explain his generation?” except God alone, who probes the secrets and returns to each one according to his works. But it is better that we simply accept that no one knows the secrets of another’s thoughts except God alone, for it says above, “Accursed is the person who trusts in humankind” and, “Blessed is the one who trusts in the Lord.” Hence, lest we think ourselves to be sure of human judgment, the psalmist implies that almost every heart is perverse, saying, “Cleanse me from hidden thoughts, O Lord, and spare your servant from f...

Lucius Caecilius Firmianus Lactantius

AD 320
David also said in the forty-fourth psalm, “Your throne, O God, is forever and ever. The scepter of your kingdom is a scepter of uprightness. You have loved righteousness. You have hated wickedness. Therefore God, your God, has anointed you with the oil of gladness.” By this word he also shows his name, since (as I have shown above) he has called Christ from his anointing. Then, that he was also man, Jeremiah teaches, saying, “And he is a man, and who shall know him?” Also Isaiah: “And God shall send to them a man who shall save them, shall save them by judging.” - "Divine Institutes 4.13"

Tertullian of Carthage

AD 220
The Father, now that he has made him a little lower than the angels, will crown him with glory and honor and will put all things beneath his feet. Then those who pierced him will know who he is and will strike their breasts, tribe to tribe—because in fact they formerly failed to recognize him in the humility of human condition. “And he is man,” says Jeremiah, “and who shall know him?” Because also, Isaiah says, “His nativity, who shall tell of it?” - "Against Marcion 3.7"
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Tertullian of Carthage

AD 220
For it is of him that Isaiah writes: “A man of suffering and acquainted with the bearing of weakness.” Jeremiah writes: “He is man, and who has known him?” And Daniel writes: “On the clouds he came as the son of man.” The apostle Paul likewise says: “The man Christ Jesus is the one mediator between God and humankind.” - "On the Flesh of Christ 15"
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Knowing this first, that no prophecy of the scripture is of any private interpretation - 2 Peter 1:20

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