3 “Prove me, O Lord, and try me”. Lest, however, any of my secret sins should be hid from me, prove me, O Lord, and try me, making me known, not to You from whom nothing is hid, but to myself, and to men. “Burn my reins and my heart.” Apply a remedial purgation, as it were fire, to my pleasures and thoughts. “For Your mercy is before my eyes”. For, that I be not consumed by that fire, not my merits, but Your mercy, whereby You have brought me on to such a life, is before my eyes. “And I have been pleasing in Your truth.” And since my own falsehood has been displeasing to me, but Your truth pleasing, I have myself been pleasing also with it and in it.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)