15 You certainly desire to be approved by God. “To everything there is a time,” as it is written: “It is time for You, Lord, to work.” “It is an acceptable time, O Lord.” You shall then make your offering when you have received permission to sacrifice, when your offering shall be acceptable to God. Would it not delight me to enjoy the favour of the Emperor, to act according to your wish, if the case allowed it? And prayer by itself is a sacrifice, it obtains pardon, when the oblation would bring offense, for the one is a sign of humility, the other of contempt. For the Word of God Himself tells us that He prefers the performance of His commandments to the offering of sacrifice. God proclaims this, Moses declares it to the people, Paul preaches it to the Gentiles. Do that which you understand is most profitable for the time. “I prefer mercy,” it is said, “rather than sacrifice.” Are they not, then, rather Christians in truth who condemn their own sin, than they who think to defend it? “The just is an accuser of himself in the beginning of his words.” He who accuses himself when he has sinned is just, not he who praises himself.
Source: Letters (New Advent)