15 That all kinds of animals are inclosed in the ark; as the Church contains all nations, which was also set forth in the vessel shown to Peter. That clean and unclean animals are in the ark; as good and bad take part in the sacraments of the Church. That the clean are in sevens, and the unclean in twos; not because the bad are fewer than the good, but because the good preserve the unity of the Spirit in the bond of peace; and the Spirit is spoken of in Scripture as having a sevenfold operation, as being "the Holy Spirit of wisdom and understanding, of counsel and might, of knowledge and piety, and of the fear of God." So also the number fifty, which is connected with the advent of the Holy Spirit, is made up of seven times seven, and one over; whence it is said, "Endeavoring to keep the unity of the Spirit in the bond of peace." The bad, again, are in twos, as being easily divided, from their tendency to schism.
That Noah, counting his family, was the eighth; because the hope of our resurrection has appeared in Christ, who rose from the dead on the eighth day, that is, on the day after the seventh, or Sabbath day. This day was the third from His passion; but in the ordinary reckoning of days, it is both the eighth and the first.
Source: Reply to Faustus the Manichaean (New Advent)