1 The comparison of the Church with Paradise shows us that men may indeed receive her baptism outside her pale, but that no one outside can either receive or retain the salvation of eternal happiness. For, as the words of Scripture testify, the streams from the fountain of Paradise flowed copiously even beyond its bounds. Record indeed is made of their names; and through what countries they flow, and that they are situated beyond the limits of Paradise, is known to all; and yet in Mesopotamia, and in Egypt, to which countries those rivers extended, there is not found that blessedness of life which is recorded in Paradise.
Accordingly, though the waters of Paradise are found beyond its boundaries, yet its happiness is in Paradise alone. So, therefore, the baptism of the Church may exist outside, but the gift of the life of happiness is found alone within the Church, which has been founded on a rock, which has received the keys of binding and loosing. "She it is alone who holds as her privilege the whole power of her Bridegroom and Lord;" by virtue of which power as bride, she can bring forth sons even of handmaids. And these, if they be not high-minded, shall be called into the lot of the inheritance; but if they be high-minded, they shall remain outside.
Source: On Baptism, Against the Donatists (New Advent)