17 And let it not be regarded as an objection to the interpretation now given, that the Apostle Peter says that Christ Himself preached to men shut up in prison who were unbelieving in the days of Noah, as if we must consider this interpretation inconsistent with the fact that at that time Christ had not come. For although he had not yet come in the flesh, as He came when afterwards He “showed Himself upon earth, and conversed with men,” nevertheless he certainly came often to this earth, from the beginning of the human race, whether to rebuke the wicked, as Cain, and before that, Adam and his wife, when they sinned, or to comfort the good, or to admonish both, so that some should to their salvation believe, others should to their condemnation refuse to believe—coming then not in the flesh but in the spirit, speaking by suitable manifestations of Himself to such persons and in such manner as seemed good to Him. As to this expression, “He came in the spirit,” surely He, as the Son of God, is a Spirit in the essence of His Deity, for that is not corporeal; but what is at any time done by the Son without the Holy Spirit, or without the Father, seeing that all the works of the Trinity are inseparable?
18. The words of Scripture which are under consideration seem to me of themselves to make this sufficiently plain to those who carefully attend to them: “For Christ has died once for our sins, the Just for the unjust, that He might bring us to God; being put to death in the flesh, but quickened in the spirit: in which also He came and preached unto the spirits in prison, who sometime were unbelieving, when the long-suffering of God waited in the days of Noah, while the ark was a preparing.” The order of the words is now, I suppose, carefully noted by you: “Christ being put to death in the flesh, but quickened in the spirit;” in which spirit He came and preached also to those spirits who had once in the days of Noah refused to believe His word; since before He came in the flesh to die for us, which He did once, He often came in the spirit, to whom He would, by visions instructing them as He would, coming to them assuredly in the same spirit in which He was quickened when He was put to death in the flesh in His passion. Now what does His being quickened in the spirit mean if not this, that the same flesh in which alone He had experienced death rose from the dead by the quickening spirit?
Source: Letters (New Advent)