Psalms 66:19

But verily God hath heard me ... - That is, He has given me evidence that he has heard my prayer; and, in doing this, he has thus given me the assurance also that I do not regard iniquity in my heart. The evidence that he has heard me is at the same time proof to my mind that I do not love sin. As it is a settled and universal principle that God does not hear prayer when there is in the heart a cherished love and purpose of iniquity, so it follows that, if there is evidence that he has heard our prayers, it is proof that he has seen that our hearts are sincere, and that we truly desire to forsake all forms of sin.