That the grace of God ought to be without price
In the Acts of the Apostles: “Your money be in perdition with yourself, because you have thought that the grace of God is possessed by money.” Also in the Gospel: “Freely you have received, freely give.” Also in the same place: “You have made my Father's house a house of merchandise; and you have made the house of prayer a den of thieves.” Also in Isaiah: “You who thirst, go to the water, and as many as have not money: go, and buy, and drink without money.” Also in the Apocalypse: “I am Alpha and Omega, the beginning and the end. I will give to him that thirsts from the fountain of the water of life freely. He who shall overcome shall possess these things, and their inheritance; and I will be his God, and he shall be my son.”
Source: The Treatises of Cyprian (New Advent)