For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit: By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ: Who is gone into heaven, and is on the right hand of God; angels and authorities and powers being made subject unto him. (1 Pet 3:18-22)
This pericope is, theologically speaking, very rich. Indeed, this passage is biblical support for baptismal regeneration, something I have discussed a few times on this blog. For instance, see the section Nicea's "We Confess One Baptism for the Remission of Sins" and Acts 2:38 at:
In his book on New Testament soteriology, Grant Macaskill (the Kirby Laing Chair of New Testament Exegesis at the University of Aberdeen) wrote the following which is rather apropos for this important issue:
Flood Typology and Participation in 1 Peter
[Flood imagery] is taken up in Peter’s christological use of the flood account in 3:18-22. This discussion is embedded into a wider section of the book in which the author speaks of the sufferings of Christians in relation to the cross. Importantly, Christ’s death ‘for sins’ (περι αμαρτιων), designated in aorist tense (επαθεν) as a particular event in time, is described as a hapax (απαξ), a one-off event. The ‘dying for’ language is taken further with another pairing in 3:18, ‘the righteous one for the unrighteous ones’, and its purpose is specified: ‘to bring you to God’ (ινα υμας προσαγαγη τω θεω). There are obvious echoes of 1 Peter 2:25 here and hence the allusion to Isaiah 53 continues to be effective. Clearly, whatever Peter will go on to say about the significance of the suffering of the individual, this is grounded upon a theology that understands the death of Jesus to be the singular ground of atonement, the one death that is in exchange for sins.
The link between the death of Jesus and the flood narrative is an important one to make. It is not simply a matter of the vindication of those who suffer: the link, rather, lies in the key term ‘flesh’ (σαρξ). Jesus, in 3:18, is put to death ‘in flesh’ but made alive ‘by the Spirit’. We have seen that in Paul’s letters, baptism represents identification with the dying and rising of Jesus, a participation in his death and resurrection. In Peter, the significance of baptism is prefigured by the flood (3:21), specifically in the element of water ‘through’ which eight people were saved. The preposition is important: the salvation spoken of is not from the waters (i.e. through the ark and providence), but through the waters. In other words, the waters are saving in effect and this can only be because they are seen to be cleansing.
It is in relation to this point that the key term ‘flesh’ is significant. In Genesis 6, that term is repeated (in 6:4 and 6:13) as the cause of the violence and iniquity that fill the earth, and the destruction of flesh is specified as the objective of the flood (6:18). Certainly, flesh will exist after the flood and the Noachic covenant is made with it (Gen 9:8-17), but there it is specified at key points that it is with ‘every living soul in all flesh’ (πασης ζωσης εν παση σαρκι). This is the kind of distinction that would allow a Jewish reader to differentiate the corrupt flesh before the flood with the living flesh that is left. The flood is, therefore, a cleansing destruction, purging the earth of that which is corrupt. This helps to explain why Peter sees it as an antitype of that which baptism symbolizes, particularly if we accept that his understanding of baptism may actually agree with that of Paul: the flood, as a purging judgement, prefigures the death of ‘flesh’ in Jesus and the establishment of a new order by the Spirit. It is worth mentioning that other literature of the time, notably The Book of Watchers, understands the flood as a cleansing and restorative event, prefiguring the eschatological judgment (particularly 1 Enoch 10, where the same theme of purging is encountered).
This requires, though, that Jesus’s identification as the sin-bearer is not simply representative but is also participatory, putting to death the old order of sins in which we used to participate and establishing a new order of righteousness in which we now participate. (Grant Macaskill, Union with Christ in the New Testament [New York: Oxford University Press, 2018], 277-78, emphasis in bold added, comment in square brackets added for clarification)