Psalm 66:18-20 If I regard iniquity in my heart, the Lord will not hear me: but verily God hath heard me; he hath attended to the voice of my prayer. Blessed be God, which hath not turned away my prayer, nor his mercy from me.
The Psalmist is confidant in faith that God has heard his prayer, paid attention to it, and that He has not turned away the answer nor His mercy. If, however, if he had regarded iniquity in his heart, that is, cherished it and held on to sin, he knew the Lord would not hear him. But he has confessed his sin, repented, and let it go. In confession and faith, he received forgiveness. “But there is forgiveness with thee, that thou mayest be feared.”
Instead of regarding sin in the heart, we respond to the promise of mercy by confessing both I am a sinner and that God, for Christ’s sake, has forgiven me. Now having a clean heart, we come to Him in the prayer of faith, knowing that the Lord hears me. The response of confession, instead of holding it in, moves me to believe the gospel and receive grace. God hears my humble, believing prayer.