20 That the raven sent out after forty days did not return, being either prevented by the water or attracted by some floating carcass; as men defiled by impure desire, and therefore eager for things outside in the world, are either baptized, or are led astray into the company of those to whom, as they are outside the ark, that is, outside the Church, baptism is destructive. That the dove when sent forth found no rest, and returned; as in the New Testament rest is not promised to the saints in this world.
The dove was sent forth after forty days, a period denoting the length of human life. When again sent forth after seven days, denoting the sevenfold operation of the Spirit, the dove brought back a fruitful olive branch; as some even who are baptized outside of the Church, if not destitute of the fatness of charity, may come after all, as it were in the evening, and be brought into the one communion by the mouth of the dove in the kiss of peace. That, when again sent forth after seven days, the dove did not return; as, at the end of the world, the rest of the saints shall no longer be in the sacrament of hope, as now, while in the communion of the Church, they drink what flowed from the side of Christ, but in the perfection of eternal safety, when the kingdom shall be delivered up to God and the Father, and when, in that unclouded contemplation of unchangeable truth, we shall no longer need natural symbols.
Source: Reply to Faustus the Manichaean (New Advent)