11 For it has been delivered to us, that there is one God, and one Christ, and one hope, and one faith, and one Church, and one baptism ordained only in the one Church, from which unity whosoever will depart must needs be found with heretics; and while he upholds them against the Church, he impugns the sacrament of the divine tradition. The sacrament of which unity we see expressed also in the Canticles, in the person of Christ, who says, “A garden enclosed is my sister, my spouse, a fountain sealed, a well of living water, a garden with the fruit of apples.” But if His Church is a garden enclosed, and a fountain sealed, how can he who is not in the Church enter into the same garden, or drink from its fountain?
Moreover, Peter himself, showing and vindicating the unity, has commanded and warned us that we cannot be saved, except by the one only baptism of one Church. “In the ark,” says he, “of Noah, few, that is, eight souls, were saved by water, as also baptism shall in like manner save you.” In how short and spiritual a summary has he set forth the sacrament of unity! For as, in that baptism of the world in which its ancient iniquity was purged away, he who was not in the ark of Noah could not be saved by water, so neither can he appear to be saved by baptism who has not been baptized in the Church which is established in the unity of the Lord according to the sacrament of the one ark.
Source: The Epistles of Cyprian (New Advent)