16 If your mind is an altar, you see whose altar it is. You may see from the very doctrines and duties in which you say you are trained. You are taught not to give food to a beggar; and so your altar smokes with the sacrifice of cruelty. Such altars the Lord destroys; for in words quoted from the law He tells us what offering pleases God: "I desire mercy, and not sacrifice." Observe on what occasion the Lord uses these words. It was when, in passing through a field, the disciples plucked the ears of grain because they were hungry. Your doctrine would lead you to call this murder. Your mind is an altar, not of God, but of lying devils, by whose doctrines the evil conscience is seared as with a hot iron, calling murder what the truth calls innocence. For in His words to the Jews, Christ by anticipation deals a fatal blow to you: "If you had known what this means, I desire mercy, and not sacrifice, you would not have condemned the guiltless."
Source: Reply to Faustus the Manichaean (New Advent)