Who can understand his errors?.... Sin is an error, a wandering out of the way of God, swerving from the rule of his word; and many mistakes are made by the people of God themselves; even so many that they cannot number them; they are more than the hairs of their head; they cannot understand, find out and express, neither their number, nor their evil nature, nor the many aggravating circumstances which attend them: this the psalmist said, upon a view of the large extent, glory, and excellency of the word of God; and upon comparing himself with it, in which, as in a glass, he saw how far short he came of it, and what a disagreement and want of conformity there was in him unto it; see Psa 119:97; and he suggests, that though the word he had been describing was perfect, pure, and clean, he was not; nor could he expect any reward of debt, but merely of grace, for his observance of it; and that it was best, under a sense of sin, to have recourse, not to works of righteousness done by men; but to the grace and mercy of God in Christ, as follows: cleanse thou me from secret faults; by which are meant not such sins as are done in secret, and are unknown to men; such as David's sin with Bathsheba, Sa2 12:12; nor the inward motions of sin in the heart, to which none are privy but God, and a man's own soul; not but that each of these may be properly enough included in such a petition; but sins, which are unknown to a man himself are meant: there are some actions, which, though known when committed, are not known to be sinful ones; and there are some sins which are committed unadvisedly, and through carelessness, and pass unobserved; not only many vain and sinful thoughts pass to and fro uncontrolled, without being taken notice of; but many foolish and idle words are spoken, and many evil actions, through infirmity and inadvertency, are done, which, when a good man, at the close of a day, comes to reflect upon the things that have passed in it, are quite hidden from him, are unknown to him, being unobserved by him; wherefore such a petition is highly proper to be inserted in his address at the throne of grace: and which also supposes the person sensible of the defiling nature of sin, and of his own impotency to cleanse himself from it; and that God only can do it, who does it by the application of the blood of his Son, which cleanses from all sin; for this respects not regenerating and sanctifying grace, but pardoning grace; a manifestation of it, a view of acquittance from sin by Christ, and of freedom from obligation to punishment for it.