2 But that the Church is one, the Holy Spirit declares in the Song of Songs, saying, in the person of Christ, “My dove, my undefiled, is one; she is the only one of her mother, she is the choice one of her that bare her.” Concerning which also He says again, “A garden enclosed is my sister, my spouse; a spring sealed up, a well of living water.” But if the spouse of Christ, which is the Church, is a garden enclosed; a thing that is closed up cannot lie open to strangers and profane persons.
And if it is a fountain sealed, he who, being placed without has no access to the spring, can neither drink thence nor be sealed. And the well also of living water, if it is one and the same within, he who is placed without cannot be quickened and sanctified from that water of which it is only granted to those who are within to make any use, or to drink. Peter also, showing this, set forth that the Church is one, and that only they who are in the Church can be baptized; and said, “In the ark of Noah, few, that is, eight souls, were saved by water; the like figure where-unto even baptism shall save you;” proving and attesting that the one ark of Noah was a type of the one Church.
If, then, in that baptism of the world thus expiated and purified, he who was not in the ark of Noah could be saved by water, he who is not in the Church to which alone baptism is granted, can also now be quickened by baptism. Moreover, too, the Apostle Paul, more openly and clearly still manifesting this same thing, writes to the Ephesians, and says, “Christ loved the Church, and gave Himself for it, that He might sanctify and cleanse it with the washing of water.” But if the Church is one which is loved by Christ, and is alone cleansed by His washing, how can he who is not in the Church be either loved by Christ, or washed and cleansed by His washing?
Source: The Epistles of Cyprian (New Advent)