11 The Lord cries aloud, that “whosoever thirsts should come and drink of the rivers of living water that flowed out of His bosom.” Whither is he to come who thirsts? Shall he come to the heretics, where there is no fountain and river of living water at all; or to the Church which is one, and is founded upon one who has received the keys of it by the Lord's voice? It is she who holds and possesses alone all the power of her spouse and Lord. In her we preside; for her honour and unity we fight; her grace, as well as her glory, we defend with faithful devotedness. We by the divine permission water the thirsting people of God; we guard the boundaries of the living fountains.
If, therefore, we hold the right of our possession, if we acknowledge the sacrament of unity, wherefore are we esteemed prevaricators against truth? Wherefore are we judged betrayers of unity? The faithful, and saving, and holy water of the Church cannot be corrupted and adulterated, as the Church herself also is uncorrupted, and chaste, and modest. If heretics are devoted to the Church and established in the Church, they may use both her baptism and her other saving benefits. But if they are not in the Church, nay more, if they act against the Church, how can they baptize with the Church's baptism?
Source: The Epistles of Cyprian (New Advent)