15 But neither must we pass over what has been necessarily remarked by you, that the Church, according to the Song of Songs, is a garden enclosed, and a fountain sealed, a paradise with the fruit of apples. They who have never entered into this garden, and have not seen the paradise planted by God the Creator, how shall they be able to afford to another the bring water of the saving lava from the fountain which is enclosed within, and sealed with a divine seal? And as the ark of Noah was nothing else than the sacrament of the Church of Christ, which then, when all without were perishing, kept those only safe who were within the ark, we are manifestly instructed to look to the unity of the Church. Even as also the Apostle Peter laid down, saying, “Thus also shall baptism in like manner make you safe;” showing that as they who were not in the ark with Noah not only were not purged and saved by water, but at once perished in that deluge; so now also, whoever are not in the Church with Christ will perish outside, unless they are converted by penitence to the only and saving lava of the Church.
Source: The Epistles of Cyprian (New Advent)