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18 For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit;
19 in which also He went and made proclamation to the spirits now in prison,
20 who once were disobedient, when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through the water.

I've heard it preached that it was the pre-incarnate Christ who preached to those in Noah's day, and now those people are in prison.

Jonathan Edwards wrote this sermon on it.

Here's a sermon by OPC minister Bill Baldwin.

And here is one by John Piper.

True godliness is a sincere feeling which loves God as Father as much as it fears and reverences Him as Lord, embraces His righteousness, and dreads offending Him worse than death~ Calvin