A) Almost everyone agrees that the expression "in which" (ε̉ν ώ) refers to "spirit" (πνεύματι) in verse 18. But they disagree about what spirit is meant here: the human spirit, that is, the soul of Christ, or the Spirit of God.
On the basis of the contrast between flesh and spirit in the preceding verse, verse 18, one must understand here the human spirit of Jesus Christ, His soul, and not the Spirit of God.
Otherwise, the descent of Jesus Christ into Hades by the Spirit of God, and not by His own soul, would in the strict sense be the descent of the Holy Spirit into Hades, and not of the Son of God.
The Church hymn says precisely this: "In the tomb bodily, in Hades with Thy soul as God, Thou wast, O Christ" (troparion at the Paschal Hours).
B) The place of the descent of Jesus Christ is called by the Apostle φυλακὴ, carcer, the prison of spirits.
From the comparison of this expression with another place in the same epistle (1 Pet 4:6), it is clear that what is meant here is the kingdom of the dead, ά̉δης, in Hebrew sheol.
To this place descended the souls of all people who died before the coming of Christ, not only the evil, but also the righteous.
At the same time, one must note that Catholic writers divide Hades itself into two halves. These are the bosom of the forefathers and Gehenna, or the place of torment of the devil, the evil spirits, and evil people.
On the basis of the Savior's parable (Lk 14:23, 26), they see between them a great gulf. Across this gulf, they suppose, even Jesus Christ Himself had no need to pass, since He descended only into the forecourt of Hades, or the bosom of the fathers.
But such an opinion contradicts the Apostle's clear indication that Jesus Christ descended into Hades.
The faith of the Church, founded on the same Holy Scripture, affirms that the Son of God descended into Hades in order to strike the devil on the head and destroy his kingdom to its foundations.
He descended to free from it all the captives who awaited His coming with faith, without distinction between good and evil. He could not have done this if He had descended only into the forecourt of Hades, the bosom of the forefathers.
The hymns of the Church present the descent of Jesus Christ into Hades precisely in this way.
"Thou didst descend into the lowest parts of the earth," the Orthodox Church sings solemnly on the day of Christ's Resurrection, "and Thou didst shatter the eternal bars that held the prisoners, O Christ" (irmos of the sixth ode of the Paschal canon).
"Today the Master has despoiled Hades, raising the prisoners whom it had held from the ages in bitter bondage" (refrain to the ninth ode of the same canon).
"Those held by the bonds of Hades, seeing Thy boundless compassion, went toward the light with joyful feet, O Christ, praising the eternal Pascha" (troparion of the fifth ode).
"We celebrate the death of death, the destruction of Hades" (troparion of the seventh ode), and the like.
C) The persons to whom the preaching is addressed are called spirits, πνεύματα. That is, they are not ordinary living people.
The designation "dead" (1 Pet 4:6) sharply separates them from the evil spirits. Their relation to those who disobeyed Noah's preaching directly points to the souls of people who perished during the flood.
The holy martyr Justin (Apolog. 2, p. 83), Clement of Alexandria (Strom. lib. 6, p. 637), and Saint Epiphanius (Haeres. 46) think that Jesus Christ descended into Hades with the preaching of His coming to the Gentiles.
They especially mean the philosophers, those who, knowing God, did not glorify Him as God (Rom 1:21). In Hades they believed in the Messiah who had descended there and were saved.
Blessed Augustine (de Genes. ad litter. L. 12, c. 33) and Saint John of Damascus (De fide orthod. 4:9) consider that He descended there to free the souls of all those tormented in Hades who were capable of receiving His preaching.
Origen (C. Cels. 2, p. 438), affirming the same opinion, even sends John the Forerunner there before Him (troparion to the Forerunner, January 7). He also sends Moses and Elijah there after the Transfiguration of Christ (Orig. hom. 4 in Luc.; Comm. in 1 Reg. et in Matth. 12).
Finally, there were interpreters who supposed that Jesus Christ led out of Hades not only those who believed in Him, but everyone who was there.
It would certainly be erroneous to suppose that all the souls of the dead who were in Hades were led out from there by Jesus Christ when He appeared in Hades with preaching to sinners.
On the other hand, one cannot allow that the preaching of the Son of God remained without effect merely because there can be no repentance after death, and because in Hades there could be neither confession (Ps 6:6) nor conversion.
In Hades, indeed, there is no repentance, but only because there is no preaching there. Yet at the time of the Son of God's descent into Hades, there was preaching.
Therefore there was also the possibility of repentance and conversion for the prisoners held in Hades.
Otherwise, there would be no reason to preach where preaching would bring no benefit to those to whom it is preached.
Nor could full victory over the devil be ascribed to Jesus Christ Himself if He had not destroyed his kingdom and freed the captives from his bonds.
The extraordinary and unique manifestation of the Son of God in Hades, His complete victory over the devil and death, and the preaching of salvation could certainly dispose even the gravest sinners to repentance.
It could compel them to bend the knee before the Victor over Hades.
This opinion is confirmed both by the fact that the Apostle calls the preaching of Jesus Christ in Hades the gospel (1 Pet 4:6), and by his indication concerning those to whom Christ preached salvation.
These were the sinners who once resisted God's longsuffering during the building of the ark and perished in the waters of the flood.
Yet probably, in the moments of their destruction during the forty-day rain, they had occasion to offer God true repentance for their disobedience to His will.
This, however, does not exclude the possibility of supposing that, just as during the preaching of the God-man on earth there were unbelievers and opponents of the truth, so in Hades there could also be sons of perdition.
There could be desperate men and mockers, who remained unbelieving and unrepentant. They trembled before the unbearable light of God's condescension and grew hardened, but did not turn to Him with a plea to be led out of the darkness of Hades.
Such as these remained in Hades together with the evil spirit, kept in eternal bonds under darkness for the judgment of the great day (Jude 6).
It remains to resolve the question why the Apostle Peter, speaking of the preaching of the Son of God in Hades, presents this preaching as addressed exclusively to the people of the first, antediluvian world.
They had not obeyed Noah's preaching. It is as though Peter forgot about other sinners who lived after the flood.
To explain this seeming strangeness, one must pay attention to the fact that the Apostle Peter distinguishes two worlds. The first ended with the flood, while the present or contemporary world is appointed for destruction by fire (2 Pet 2:5; 2 Pet 3:6, 7).
He joins all postdiluvian people into one world, which still continues to exist. In the few persons contemporary with Jesus Christ and the Apostles, this world heard the preaching of salvation from the mouth of the Savior Himself.
Before His coming, it heard this preaching from the prophets, who preached by the same Spirit of Christ.
But all who lived before the flood belong to the first world. This world, separated from Jesus Christ by whole millennia, was not deprived of this preaching.
Jesus Christ even descended into Hades in order to preach to it about His coming.
Thus the Apostle had in mind here to speak about the universality of the preaching of salvation. He meant its extension even to the first world, which seemed least of all able to hear this preaching.
At the same time, the Apostle wishes to indicate the Lord's boundless compassion. Even in the most rejected and destroyed world, He sees at least small remnants of good.
He sends His Son to lead out from Hades itself those who, in the waters of the flood, washed away their sin of resistance to God's will by bringing repentance for it.