In what the true fasting consists
Justin: Learn, therefore, to keep the true fast of God, as Isaiah says, that you may please God. Isaiah has cried thus:
Shout vehemently, and do not spare: lift up your voice as with a trumpet, and show My people their transgressions, and the house of Jacob their sins. They seek Me from day to day, and desire to know My ways, as a nation that did righteousness, and forsook not the judgment of God. They ask of Me now righteous judgment, and desire to draw near to God, saying, Wherefore have we fasted, and You see not? And afflicted our souls, and You have not known? Because in the days of your fasting you find your own pleasure, and oppress all those who are subject to you. Behold, you fast for strifes and debates, and smite the humble with your fists. Why do you fast for Me, as today, so that your voice is heard aloud? This is not the fast which I have chosen, the day in which a man shall afflict his soul. And not even if you bend your neck like a ring, or clothe yourself in sackcloth and ashes, shall you call this a fast, and a day acceptable to the Lord. This is not the fast which I have chosen, says the Lord; but loose every unrighteous bond, dissolve the terms of wrongous covenants, let the oppressed go free, and avoid every iniquitous contract. Deal your bread to the hungry, and lead the homeless poor under your dwelling; if you see the naked, clothe him; and do not hide yourself from your own flesh. Then shall your light break forth as the morning, and your garments shall rise up quickly: and your righteousness shall go before you, and the glory of God shall envelope you. Then shall you cry, and the Lord shall hear you: while you are speaking, He will say, Behold, I am here. And if you take away from you the yoke, and the stretching out of the hand, and the word of murmuring; and shall give heartily your bread to the hungry, and shall satisfy the afflicted soul; then shall your light arise in the darkness, and your darkness shall be as the noon-day: and your God shall be with you continually, and you shall be satisfied according as your soul desires, and your bones shall become fat, and shall be as a watered garden, and as a fountain of water, or as a land where water fails not.
“Circumcise, therefore, the foreskin of your heart,” as the words of God in all these passages demand.
Source: Dialogue with Trypho (New Advent)