21 There are many other points which we cannot take notice of even in this cursory manner. Why in the six hundred and first year of Noah's life— that is, after six hundred years were completed— the covering of the ark is removed, and the hidden mystery, as it were, disclosed. Why the earth is said to have dried on the twenty-seventh day of the second month; as if the number fifty-seven denoted the completion of the rite of baptism. For the twenty-seventh day of the second month is the fifty-seventh day of the year; and the number fifty-seven is seven times eight, which are the numbers of the spirit and the body, with one over, to denote the bond of unity.
Why they leave the ark together, though they entered separately. For it is said: "Noah went in, and his sons, and his wife, and his sons' wives with him, into the ark;" the men and the women being spoken of separately; which denotes the time when the flesh lusts against the spirit, and the spirit against the flesh. But they go forth, Noah and his wife, and his sons and their wives—the men and women together. For in the end of the world, and in the resurrection of the just, the body will be united to the spirit in perfect harmony, undisturbed by the wants and the passions of mortality. Why, after leaving the ark, only clean animals are offered in sacrifice to God, though both clean and unclean were in the ark.
Source: Reply to Faustus the Manichaean (New Advent)