Theophylact of Ohrid — on 1Pet 3:18-19 (Commentary on 1 Peter)

“Having been put to death "in the flesh," that is, as a man; but made alive "by the Spirit," as God. By the word "Spirit" is meant God, and by "flesh" – man. The evangelist testifies to the first when he relates that Christ, the true Wisdom, speaking with the Samaritan woman, said: "God is a spirit" (John 4:24), and all of Holy Scripture testifies to the second. From this it is clear that Christ is twofold (dual) not in Person, but in nature. "By which" is used instead of "wherefore." For having said that He died for us, the unrighteous, the apostle then says that He also preached to those held in Hades. Having reached this point, the apostle still had to explain how the death of Christ was beneficial for those who had previously fallen asleep, and to resolve the question: if the incarnation of the Lord was for the salvation of all, then what salvation did those who had previously died receive? He immediately resolves both matters and says that the death of Christ accomplished both together: the hope of resurrection through His resurrection, and the salvation of those who had previously fallen asleep. For those who had spent the time of their life well received salvation even then through the descent of the Lord into Hades, as Saint Gregory thinks. He says: "Christ, having appeared to those in Hades, saves not all without exception, but only the believers." For it depended on the free will of each person (as reason demanded) not to remain insensible to the rich gift of the Creator, but to show oneself worthy of the goodness of the Giver.”