David, the author of this majestic psalm of praise to God for His revelation of Himself and His nature to man, voices his own frustration at his inability to mold his life totally in accordance with God's revealed plan. He recognizes and asks for God's forgiveness for his failure to measure up, and asks for strength to avoid habitual sin patterns and willful rejection of God's way.
God had already made a careful distinction between these types of sins. "The priest shall make an atonement for the soul that sinneth ignorantly, when he sinneth by ignorance before the LORD, to make an atonement for him; and it shall be forgiven him. . . . But the soul that doeth ought presumptuously . . . the same reproacheth the LORD; and that soul shall be cut off from among his people. Because he hath despised the word of the LORD, and hath broken his commandment, that soul shall utterly be cut off; his iniquity shall be upon him" (Numbers 15:28, 30-31).
Paul also recognized such a difference. Keep in mind that all sin is abhorrent to God and must be repented of, resulting, of course, in His forgiveness. But Paul claimed that even his blasphemous, murderous persecution of the church was done "ignorantly in unbelief" (1 Timothy 1:13). His plea of ignorance did not excuse his guilt, but through it he "obtained mercy" (v. 13) and "grace" (v. 14).
This is a "pattern to |us| which should hereafter believe on him to life everlasting" (v. 16). Let us not be guilty of willful, presumptuous sin, but on these occasions when we do fall, we can be thankful that our "longsuffering" (v. 16) Savior still affords us such mercy. JDM