Albert Barnes Commentary Psalms 66:18

"If I regard iniquity in my heart, The Lord will not hear:" — Psalms 66:18 (ASV)

If I regard iniquity in my heart - literally, “If I have seen iniquity in my heart.” This means if I have indulged in a purpose of iniquity, if I have had a wicked end in view, if I have not been willing to forsake all sin, or if I have cherished a purpose of pollution or wrong.

The meaning is not literally, if I have “seen” any iniquity in my heart—for no one can look into his own heart and not see that it is defiled by sin. Rather, it means if I have cherished sin in my soul, if I have gloated over past sins, if I am purposing to commit sin again, or if I am not willing to abandon all sin and to be holy.

The Lord will not hear me - This means He will not regard and answer my prayer. The idea is that for prayer to be heard, there must be a purpose to forsake all forms of sin.

This is a great and most important principle regarding prayer. The same principle is affirmed or implied in Psalm 18:41; Psalms 34:15; Proverbs 1:28; Proverbs 15:29; Proverbs 28:9; Isaiah 15:1–9; Jeremiah 11:11; Jeremiah 14:12; Zechariah 7:13; and John 9:31. It is also especially stated in Isaiah 58:3-7.

The principle is applicable:

1. to secret purposes of sin; to sinful desires, corrupt passions, and evil propensities;
2. to acts of sin in individuals, as when a man is pursuing a business founded on fraud, dishonesty, oppression, and wrong;
3. to public acts of sin, as when a people fast and pray (Isaiah 58:1–14), and yet hold their fellow-men in bondage, or enact and maintain unjust and unrighteous laws, or uphold the acts of wicked rulers, or countenance and support by law that which is contrary to the law of God;
4. to the feelings of an awakened and trembling sinner when he is professedly seeking salvation.

If there is still the love of evil in his heart; if he has some cherished purpose of iniquity which he is not willing to abandon; if there is any one sin, however small or unimportant it may seem to be, which he is not willing to forsake, he cannot hope that God will hear his prayer; he may be assured that He will not.

All prayer, to be acceptable to God, must be connected with a purpose to forsake all sin.