Augustine of Hippo — on 1Pet 3:18-19 (Letter 164 (A.D. 414), Sections 2, 3, 6, 8, 10)

“After having said that "Christ was put to death in the flesh, and quickened in the spirit," the apostle immediately went on to say: "in which also He went and preached unto the spirits in prison; which sometime were unbelieving, when once the long-suffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water;" thereafter he added the words: "which baptism also now by a like figure has saved you." [1 Peter 3:18-21] This, therefore, is felt by me to be difficult. If the Lord when He died preached in hell to spirits in prison, why were those who continued unbelieving while the ark was a preparing the only ones counted worthy of this favour, namely, the Lord's descending into hell? For in the ages between the time of Noah and the passion of Christ, there died many thousands of so many nations whom He might have found in hell. I do not, of course, speak here of those who in that period of time had believed in God, as, e.g. the prophets and patriarchs of Abraham's line, or, going farther back, Noah himself and his house, who had been saved by water (excepting perhaps the one son, who afterwards was rejected), and, in addition to these, all others outside of the posterity of Jacob who were believers in God, such as Job, the citizens of Nineveh, and any others, whether mentioned in Scripture or existing unknown to us in the vast human family at any time. I speak only of those many thousands of men who, ignorant of God and devoted to the worship of devils or of idols, had passed out of this life from the time of Noah to the passion of Christ. How was it that Christ, finding these in hell, did not preach to them, but preached only to those who were unbelieving in the days of Noah when the ark was a preparing? Or if he preached to all, why has Peter mentioned only these, and passed over the innumerable multitude of others? It is established beyond question that the Lord, after He had been put to death in the flesh, "descended into hell;" for it is impossible to gainsay either that utterance of prophecy, "You will not leave my soul in hell," — an utterance which Peter himself expounds in the Acts of the Apostles, lest any one should venture to put upon it another interpretation — or the words of the same apostle, in which he affirms that the Lord "loosed the pains of hell, in which it was not possible for Him to be holden." Who, therefore, except an infidel, will deny that Christ was in hell? As to the difficulty which is found in reconciling the statement that the pains of hell were loosed by Him, with the fact that He had never begun to be in these pains as in bonds, and did not so loose them as if He had broken off chains by which He had been bound, this is easily removed when we understand that they were loosed in the same way as the snares of huntsmen may be loosed to prevent their holding, not because they have taken hold. It may also be understood as teaching us to believe Him to have loosed those pains which could not possibly hold Him, but which were holding those to whom He had resolved to grant deliverance... As to the first man, the father of mankind, it is agreed by almost the entire Church that the Lord loosed him from that prison; a tenet which must be believed to have been accepted not without reason, — from whatever source it was handed down to the Church — although the authority of the canonical Scriptures cannot be brought forward as speaking expressly in its support, though this seems to be the opinion which is more than any other borne out by these words in the book of Wisdom. [Wisdom 10:1-2] Some add to this [tradition] that the same favour was bestowed on the holy men of antiquity — on Abel, Seth, Noah and his house, Abraham, Isaac, Jacob, and the other patriarchs and prophets, they also being loosed from those pains at the time when the Lord descended into hell... But seeing that plain scriptural testimonies make mention of hell and its pains, no reason can be alleged for believing that He who is the Saviour went there, except that He might save from its pains; but whether He did save all whom He found held in them, or some whom He judged worthy of that favour, I still ask: that He was, however, in hell, and that He conferred this benefit on persons subjected to these pains, I do not doubt... You perceive, therefore, how intricate is the question why Peter chose to mention, as persons to whom, when shut up in prison, the gospel was preached, those only who were unbelieving in the days of Noah when the ark was a preparing — and also the difficulties which prevent me from pronouncing any definite opinion on the subject.”