1 Peter 3:18-20 -- Explanation of Christ preaching to the souls in prison:

For Christ...was put to death in the body but made alive by the Spirit, through whom also He went and preached to the spirits in prison
who disobeyed long ago when God waited patiently in the days of Noah while the ark was being built. (1 Peter 3:18-20).

Who are these "spirits in prison," and how and when did Jesus Christ "preach" unto them? Many different explanations have been offered (including Christ preaching to souls in Hades between the time of His death and resurrection), but perhaps the most simple and convincing explanation is that it was the Spirit of Christ who preached the message of salvation through His servant Noah unto the people of Noah's day, and that this proclamation occurred during those years prior to the flood. This was the view of St. Augustine (AD 400), and it is the interpretation embraced by many scholars today.

We know that Noah was "a preacher of righteousness" (2 Peter 2:5), so we know that these lost beings (bound and imprisoned in sin) were having the message of salvation proclaimed to them through his efforts. We also know that the OT proclaimers were said to be preaching their message to the lost people about them by means of "the Spirit of Christ within them" (1 Peter 1:10-11).

Therefore, Peter, in the context of the very book wherein we find our difficult passage, confirms for us that the "Spirit of Christ" was proclaiming the "good news" through the OT spokesmen of God. And among those OT proclaimers, according to Peter, was Noah. Thus, Christ was preaching to those people before their physical deaths, prior to the coming of the flood, through Noah.

Well known protestant expositors holding this view include John Wesley and Jamieson, Fausset & Brown. Adam Clarke stated that it was "by the ministry of Noah" that the Spirit of Christ preached to "the inhabitants of the antediluvian world" (Clarke's Commentary, vol. 6, p. 861). The "prison" mentioned in the passage is the imprisonment in sin and under the curse, as in verses like Isaiah 61:1, Luke 4:18, Gal 3:23). It is not uncommon for the Hebrew bible writers to refer to human persons as "souls" and even "spirits" (Num 27:16).