Charles H. Spurgeon on Psalms 78:57

They were turned aside like a deceitful bow. When the bow is unbent the rift it hath may be undiscerned, but go to use it by drawing the arrow to the head, and it flies in pieces; thus doth a false heart when put to the trial. As the ape in the fable, dressed like a man, when nuts are thrown before her, cannot then dissemble her nature any longer, but shows herself as ape indeed; a false heart betrays itself before it is aware, when a fair occasion is presented for its lust; whereas sincerity keeps the soul pure in the face of temptation. William Gurnall.

Verses 57. The fourth thing is the deceitful bow, (hymr tvq), a slack or warping bow arcus doli vel dolosus seu fallax (Hebrew) will be sure to deceive the archer that shoots in it; it will turn back into belly, as the archer's phrase is; and though he level both his eye and his arrow never so directly to the mark and think confidently with himself to hit it; yet, in the event, the arrow, through the warping of the bow, flies a quite contrary way, yea, and sometimes reflects upon the archer himself. Non semper feriet, quodcunque minabitur arcus, the bow smites not all it threatens, and the ill fashioned or casting bow will turn in the shooter's hand, and send the arrow sometimes one way and sometimes another way; yea, and sometimes it rebounds into his own sides; or if it be a rotten bow (though otherwise fair to look upon), when an arrow is drawn to the head it breaks in the hand, and deceives the archer. The same thing happeneth when the string of the bow is naughty, and breaks when the arrow is drawn. This is no less than a divine Scripture allegory. Behold, such a fallacious, warping, and rotten bow is man's deceitful heart; his purposes and promises are the arrows that he puts upon the string, the mark he aims at is repentance, to the which (in affliction especially) he looketh with an accurate and intent eye, as though he would repent indeed; but, alas! his heart deceives him, as being unsound in God's statutes, Ps 119:80; and hence it is that his promises and pretences do fall at his foot, or vanish in the air as smoke. Thus a deceiving, as well as a deceived, heart, turns him aside, Isa 64:20, as it did those false Israelites: oh, then, look to the secret warping of your own heart, and seeing you are God's bow, you must be bent by him, and stand bent for him, Zec 9:13; thereby you shall be like Jonathan's bow that "never returned empty, "2Sa 1:22. Christopher Ness, in "A Crystal Mirror." 1679.

Verses 57-59. Not to be settled in the faith, is provoking to God. To espouse the truth, and then to fall away, brings an ill report upon the gospel, which will not go unpunished. They turned back, and dealt unfaithfully. When God heard this, he was wroth, and greatly abhorred Israel. The apostate drops as a windfall into the devil's mouth. Thomas Watson.