I Pt 3.19 is a difficult verse on its face, but it’s easily explainable. It’s tied to 3.18, “His physical body was killed, but he was brought back to life in spirit.” 3.18 simply says that he also used God’s power to help Noah tell everyone that a global flood was imminent. All but 8 people were killed in the flood because they refused to listen. Those “spirits now in prison” are identified clearly as those who lived in the antediluvian world.
Interestingly, many megalithic structures around the world (Ollantaytambo, Barabar, Ajanta, Ellura caves, Puma Punku, the city of Petra, Derinkuyu, Saqsaywaman, the Great Pyramid of Giza, etc.) show strong evidence of having been built before the flood. Several of these seem to have been designed specifically to protect against a doomsday scenario. We know all people were warned about the flood, but it seems most preferred to put their trust in stone rather than a wooden boat.
3.21 is the crux of the matter. Just as Noah and his family were saved from evil because of the flood, we get a clean conscience through baptism. We only have hope because Jesus came back to life, proving that death is on borrowed time. If we want a life with Jesus in a perfect world after we die, we have to make a formal appeal to God for a clean conscience. According to this verse, the only way to make that appeal is through baptism.
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