And I wept, my soul fasted David here proves, by the signs or effects, that his efforts to promote the Divine glory proceeded from a pure and well-regulated zeal, inasmuch as he was not impelled or inflamed by the impetuosity of the flesh, but rather humbly abased himself before God, choosing him to be the witness of his sorrow. By this he shows the more evidently the incorrigible perversity of his enemies. It frequently happens, that those who set themselves boldly for the vindication of the glory of God, provoke and exasperate the wicked to a higher pitch by opposing them contentiously and without moderation. But David's zeal was so tempered that it ought to have softened even the hardness of steel. By this circumstance he, however, intended to show that he was oppressed with such violence by the frowardness of his enemies, that he dared not even open his mouth to speak a single word in defense of the cause of God, and no other means were left him of defending it but tears and mourning. He was deprived, as we know, of the liberty of giving utterance to the sentiments of his heart, or rather his words, as being those of a condemned person, would have been repelled with cruel reproaches. It was a proof of the greater constancy when in such circumstances he continued to burn with a zeal as unabated as ever, and persevered in the voluntary sorrow which he had engaged to exercise with the view of maintaining the honor and glory of God. He accordingly declares, that he wept and that his soul fasted, and that he was clothed with sackcloth; which were the tokens of mourning among the Jews. But his enemies turned all these things into mockery and jesting; [1] from which it is manifest that they were carried away with the fury of demons. It is of importance for us to be fortified with such an example, that in the present day we may not be discouraged when we meet with the same perversity by which the enemies of the Gospel prove themselves to be rather devils than men. We must, however, beware of pouring oil upon the fire which is already burning too fiercely, and should rather imitate David and Lot, who, although they had not liberty to rebuke the wicked, were yet deeply grieved in their hearts. And even when the wicked are constrained to hear us, mildness and humility will be a powerful means, or rather will be the best seasoning, for tempering holy zeal. Those who conceive of David as intimating that he resigned himself to suffer punishment in the room or stead of his enemies, attempt to confirm their opinion from his having clothed himself in sackcloth. But I take it more simply as meaning, that when he saw things in such a state of confusion, he voluntarily engaged in this sorrowful exercise to testify that nothing was more grievous to him than to witness the sacred name of God exposed to contumely.

Footnotes

1 - "That was turned to my reproach; i.e., it was made a subject of reproach to me." -- Cresswell.

When I wept, and chastened my soul with fasting - The words "and chastened" are not in the original. The literal translation would be, "And I wept (away) my soul with fasting;" that is, I gave myself so much to fasting accompanied with weeping, that my strength was exhausted. This refers to his acts of devotion; to his endeavors to discipline his soul so as to lead a strictly religious life.
That was to my reproach - This may either mean that they accused him of hypocrisy and insincerity; or, that they charged him with folly for being so religious, so strict, so self-sacrificing, so serious - perhaps they would say, so superstitious, so gloomy, so fanatical. The latter best accords with the connection, since it was for his "religion" mainly that they reproached him, Psalm 69:7-9.

When I (k) wept, [and chastened] my soul with fasting, that was to my reproach.
(k) My zeal moved me to lament and pray for my salvation.

When I wept,.... Because of the sins of his people imputed to him; the hardness and unbelief of the Jews that rejected him; their impiety and profaneness in polluting the temple with their merchandise: he wept at the grave of Lazarus, and over the city of Jerusalem, on account of the blindness of its inhabitants, and the ruin coming upon them; and in his prayers at different times, especially in the garden and on the cross, which were offered up with strong crying and tears; see John 11:35;
and chastened my soul with fasting; or "my soul being in fasting" (y). The Targum renders it, "in the fasting of my soul"; the word "chastened" is supplied from Psalm 35:13; and "soul" is put for the body, or for the whole person. Christ fasted forty days and nights in the wilderness; and often, through neglect of himself, and multiplicity of business, in preaching, and in healing diseases, was without food for some time: he seems to have been fasting the day that he suffered, when he made atonement for sin; and so answered the type on the day of atonement, when every man was to afflict his soul with fasting, Leviticus 16:29; hence the Jews taunting at him gave him gall for his meat, and vinegar for his drink, Psalm 69:21; and it follows,
that was to my reproach; if he ate and drank, he was charged with being a glutton and a winebibber; and if he wept and fasted, as John his forerunner did, they reproached him with madness, and having a devil, Matthew 11:18; and, as may be reasonably supposed, after this manner;
"can this poor creature, that weeps, and mourns, and fasts, be thought to be the Son of God, a divine Person, as he makes himself to be, and his followers believe he is?''
and so the blind Jews reason to this day.
(y) "cum esset in jejunio anima mea", Musculus, Cocceius, Gejerus, De Dieu.

wept (and chastened) my soul--literally, "wept away my soul," a strongly figurative description of deep grief.

Wept - For their impiety. Reproach - They derided me for it.