Here the psalmist expects to fall into errors and unconscious faults, and he prays to be cleansed from them, but he prays to be kept from known and voluntary sins.
Hence it is evident that sins are incompatible with David's idea of perfection; and that unnoticed and involuntary errors or faults, are not. This distinction is strongly confirmed by an inquiry into the facts of David's life, and God's verdict respecting his character. In I Kings xv. 5, we are assured that he "did that which was right in the eyes of the Lord, and turned not aside from any thing that He commanded him, all the days of his life, save only in the matter of Uriah, the Hittite." From all "presumptuous sins," save one, David was kept. Notwithstanding his infirmities, he did that which was right in the eyes of the Lord, with one sad and solitary exception.
But, when God sums up the life and character of King Asa, he makes no exception to his perfectness, declaring that "the heart of Asa was perfect all his days" (2 Chron. xv. 17). Yet we find that he failed to perfect his reform by taking away all the high places of idolatrous worship: that he was angry with Hanani, who rebuked him for his lack of trust in God against Baasha, King of Israel, and that he put him in prison, and oppressed some of the people, who were probably regarded as factious and disloyal in their sympathy with the imprisoned prophet, whose rectitude of purpose Asa had entirely, yet innocently misapprehended. In addition, the sacred historian has recorded another infirmity, common with some of the holiest men now on the earth, who employ physicians for bodily ailments, and doubt that the gift of healing is still available — "In his disease he sought not to the Lord, but to the physicians" (2 Chron. xvi. 10-12). Doubtless, many of his contemporaries saw great imperfections in these outward acts, these mistaken judgments and severities in administration, but the Lord, who looks at the heart, chisels on Asa's tombstone this enviable epitaph, "Perfect all his days." We aspire to no better. Is it impossible for us to achieve under the Gospel what it was possible to accomplish under Judaism? If so, what has Christ procured, and what has the Holy Spirit bestowed, which should make His dispensation more glorious?
When we look into the Gospel we find Jesus Christ making [this] very distinction... Of the traitor who willfully betrayed Him, He said. "It had been good for that man if he had not been born;" but to the sleeping disciples in Gethsemane He hinted no destiny of remediless woe in these tender words. "The spirit indeed is willing, but the flesh is weak." Judas had sinned; Peter, James, and John had been overcome by an infirmity. Paul makes the same distinction in these two precepts, "Them that sin, rebuke before all, that others may fear" (I Tim. v. 20). "We, that are strong, ought to bear the infirmities of the weak" (Rom. xv. 1).
The moral sense of mankind makes a distinction not in degree, but in kind, between forging a note, and falling asleep in a prayer meeting, or forgetting to keep a promise, or disproportioning food to exercise, or indulging too long in sleep, or having an impure dream, or a wandering thought in church, or treating a neighbor coldly under a misapprehension of his worthiness. The universal conscience discriminates between a sin and a weakness or an error.
— Edited from Mile-Stone Papers, Part 1, Chapter 7.