“Let the words of my mouth and the meditation of my heart be acceptable in your sight, O Lord, my rock and my redeemer.” -Psalm 19:14
David knew his words were the fruit of his heart’s meditations, and to be acceptable in the Lord’s sight, he had to mortify those sins as well. In other words, it’s not only the external transgressions for which we need to seek the Lord’s help; it’s also with the internal sins that we need help. These will condemn us and destroy us equally as fast as the others, though they are less noticeable.
And this is something Jesus warned his disciples about. The Jews of his day had reduced the Law to specific external acts, hiding their sin in facile distinctions of their man-made doctrines. But, in his famous Sermon on the Mount, Jesus repeatedly corrected this wrong thinking, saying, “You have heard that it was said to those of old, You shall not [enter sin of the body], But I say to you [enter the sin of the heart]” (Matthew 5:21-48).
In other words, David knew the truth that Jesus taught, “The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks” (Luke 6:45), and prayed his Rock and Redeemer would set him free and keep him from those sins.