Charles Haddon Spurgeon on Psalm 66:18–
If I regard iniquity in my heart. If, having seen it to be there, I continue to gaze upon it without aversion; if I cherish it, have a side glance of love toward it, excuse it, and palliate it; the Lord will not hear me. How can he? Can I desire him to connive at my sin, and accept me while I wilfully cling to any evil way? Nothing hinders prayer like iniquity harboured in the breast; as with Cain, so with us, sin lieth at the door, and blocks the passage. If thou listen to the devil, God will not listen to thee. If you refuse to hear God’s commands, he will surely refuse to hear thy prayers. An imperfect petition God will hear for Christ’s sake, but not one which is wilfully miswritten by a traitor’s hand. For God to accept our devotions, while we are delighting in sin, would be to make himself the God of hypocrites, which is a fitter name for Satan than for the Holy One of Israel.