1 The question which you have proposed to me from the epistle of the Apostle Peter is one which, as I think you are aware, is wont to perplex me most seriously, namely, how the words which you have quoted are to be understood on the supposition that they were spoken concerning hell? I therefore refer this question back to yourself, that if either you yourself be able, or can find any other person who is able to do so, you may remove and terminate my perplexities on the subject. If the Lord grant to me ability to understand the words before you do, and it be in my power to impart what I receive from Him to you, I will not withhold it from a friend so truly loved. In the meantime, I will communicate to you the things in the passage which occasion difficulty to me, that, keeping in view these remarks on the words of the apostle, you may either exercise your own thoughts on them, or consult any one whom you find competent to pronounce an opinion.
2. 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.” 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?
Source: Letters (New Advent)