In verses 18-22 Peter speaks of Christ's descent into hell, which took on so pronounced an emphasis in Christian faith and worship that it became an article in the Nicene Creed. Peter says that Christ "went and preached to the spirits in prison, who formerly were disobedient, when once the Divine longsuffering waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight souls, were saved through water. There is also an antitype which now saves us—baptism (not the removal of the filth of the flesh, but the answer of a good conscience toward God), through the resurrection of Jesus Christ."
The relationship of Christian Baptism to the Flood and Noah's Ark, found here explicitly for the first time, became a common trope in Christian biblical exegesis:
Righteous Noah, along with the other mortals at the Deluge, that is, with his own wife, with his three sons, and with their three wives, all of them being eight in number, were a symbol of the eighth day, whereon Christ appeared when He rose from thee dead, first in power forever. For Christ, being the firstborn of every creature, became again the head of another race regenerated by Himself through water, and faith, and wood, containing the mystery of Cross, even as Noah was saved by wood when he rode upon the waters with his family (Justin Martyr, Dialogue With Trypho 138).
Just as the waters of the Deluge, by which the old iniquity was purged—after the baptism of the world, so to speak—a dove became the herald announcing to thee earth the softening of the heavenly wrath, when she had been sent away out of the Ark, and had returned carrying the olive branch, a sign that even among the pagans signifies peace, so by the selfsame law of the heavenly dispensation, there flies to the earth—that is to say, our flesh—as it emerges from the font, having put away its old sins, the dove of the Holy Spirit, bringing us the peace of God, sent forth from heaven, where is the Church, typified by the Ark (Tertullian, On Baptism 8).