Who can say, I have made my heart clean, I am pure from my sin? The question implies the answer, "No one." This is expressed in Job 14:4, "Who can bring a clean thing out of an unclean? not one." At the dedication of the temple, Solomon enunciates this fact of man's corruption, "There is no man that sinneth not" (1 Kings 8:46).
The prophet testifies, "The heart is deceitful above all things, and it is desperately sick: who can know it?" (Jeremiah 17:9). And St. John warns, "If we say that we have no sin, we deceive ourselves, and the truth is not in us" (1 John 1:8).
The heart is cleansed by self-examination and repentance; but it is so easy to deceive one's self in this matter, sins may lurk undetected, motives may be overlooked, so that no one can rightly be self-righteous, or conceited, or proud of his spiritual state.
The "my sin" at the end of the clause is rather possible than actual sin; and the expression means that no one can pride himself on being secure from yielding to temptation, however clean for a time his conscience may be.
The verse, therefore, offers a stern corrective of two grievous spiritual errors—presumption and apathy.