John Trapp on Jeremiah 17:9

Ver. 9. The heart of man is deceitful above all things] The pravity and perverſity of mans heart, full of harlotry and creature-confidence, deceiving and being deceived, is here plainly and plentifully deſcribed; and oh that it were duely and deeply conſidered. Deceitful it is here ſaid to be above all things: no creature like it. Varium eſt, verſutum, & verſipelle; tortuoſum eſt, anfractuoſum & fallax, ideoque inſcrutabile. It is full of turnings and windings, nooks and corners, wiles and ſlights. It deceived David (as wiſe as he was) and tripped up his heels (as the word here uſed importeth) Pſal. 39.1, 2, 3. ſo it did Peter, Joh. 13.37, 38. Fitly doth the Prophet here call our hearts deceitful, in that word in the Original, from whence Jacob had his name▪ becauſe our fleſhly hearts do the ſame things to the ſpirit in doing of good, which Jacob did to his brother; ſupplant it, and catch it by the heele, while it is running the Chriſtian race. As Jehu offered ſacrifice to Baal, killing his Prieſts at the ſame time (and this he did in ſubtilty to circumvent them,See Dike of the deceitfulneſs of the Heart. 2 King. 10.19.) and as Huſhai went to Abſoloms company to overthrow him: ſo deale our deceitful hearts with us, [non-Latin text not transcribed]. &c. Neither is it deceitful only, but deep (ſo the Septuagint here render it) thoſe that are ſtill digging in this dunghil, do find it to be a very bottomleſſe pit. Yea it is,

Desperately wicked] Deſperately bent upon deadly miſchief. So that he gave no evil counſel who ſaid to his friend, Ita cave tibi, ut caveas teipſum: ſo ſee to thy ſelf, that thou beware of thine own heart. Another prayed not amiſs, Lord keep me from that naughty man, my ſelf. Take heed of the devil and the world (ſaid a certain Martyr in a letter to his wife) but eſpecially of thine own heart: — Non longe ſcilicet hoſtes Quaerendi nobis, circumſtant undique muros. We have a Trojan horſe full of armed enemies in the cittadel of our hearts. We have Jebuſites enough within us to undo us, [non-Latin text not transcribed]. Enchirid. cap. 72. quos nec fugere poſſumus nec fugare. It was no ill character therefore of a good man that is given by Epictetus a Heathen, that he carefully watcheth himſelf as his own deadly enemy.

Who can know it?] None but a mans ſelf, 1 Cor. 2.11. nor yet a mans ſelf neither; for nothing is more common then ſelf-deceit, Gal. 6.3. Jam. 1.21. How much was Bellarmine (that great Scholler) miſtaken? and how ill read in his own heart, when as the Prieſt coming to abſolve him on his death-bed, he could not remember any particular ſin to confeſſe, till he went back in his thoughts as far as his youth? had he but thruſt his hand into his own boſom with Moſes, he had brought it out leprous, white as ſnow. Had he looked well into his own heart, he would have found it to be a raging ſea of ſin, Iſa. 57.20. where is that Leviathan (the devil) beſides creeping things (crawling luſts) innumerable. This made bleſſed Bradford never look on any mans lewd life, but he would ſtrait cry out, Lord have mercy upon me: for in this my vile heart remaineth that ſin, which without Gods ſpecial grace, I ſhould have committed as well as he.