The harrowing of hell

18 For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive by the Spirit, 19 in which he went and proclaimed to the spirits in prison, 20 because they formerly did not obey, when God's patience waited in the days of Noah, while the ark was being prepared (1 Pet 3:18-20).

This is an obscure passage that's generated several competing interpretations. 

1. In the person of Noah, the preexistent Son commanded Noah's contemporaries to repent.

i) A basic deficiency of that interpretation is that the passage doesn't say or imply that Jesus spoke through Noah. 

ii) This interpretation depends on rendering the dative pneumati in locative terms ("in the spirit") rather than instrumental terms ("by the Spirit"). Hence, Christ spoke through Noah via the intermediate agency of the Spirit. 

However, the distinction between "put to death" and "made alive" alludes to the crucifixion and Resurrection respectively. Jesus was raised by the agency of the Holy Spirit. Given the conceptual contrast between physical death and physical resurrection, it makes more contextual sense to render pneumati in instrumental terms ("by the Spirit"). Otherwise, we have a Docetic/Gnostic Resurrection. 

iii) In defense of (1), it dovetails with 1 Pet 1:11, with its reference to the prophetic "Spirit of Christ." However:

a) That's not what 3:18-20 say. Even if it's consistent with 3:18-20, nothing in vv18-20 which indicates that referent.

b) 1:11 has its own interpretive issues. What exactly is mean by the "Spirit of Christ"? Is Christ the subject? Did he take possession of OT prophets?

Or is Christ the object? Is he the topic of OT prophecy? In context, it refers to prophecies about Christ rather than prophecies by Christ. It is not through the agency or instrumentality of Christ, but the Spirit of God.

iii) The sequence of the passage suggests this took place after the Resurrection, and not in prediluvian times:

death>Resurrection>Ascension (v22).

2. During Holy Saturday (between Good Friday and Easter Sunday), Christ went to the limbus patrum to release the OT saints from Purgatory. This is the traditional "descent into hell" or "harrowing of hell."

i) Aside from the anachronism (see above), this assumes the dogma of Purgatory. But that's hardly something an exegete can take for granted. 

ii) Likewise, "spirit" is not a synonym for the discarnate soul of Christ. How could that be "made alive" at the moment of death?  

3. After the Resurrection, Christ proclaimed final condemnation to imprisoned angels who fell in the days of Noah. A variation on this view refers it to the souls of their offspring (Nephilim), whom they begat with women. 

The subjection of angels to Christ in v22 supports this interpretation. The "spirits" in v19 are the same as the beings in v22.

i) This typically assumes that Peter is alluding to 1 Enoch's interpretation of Gen 6:1-4. The imprisoned "spirits" are the fallen angels. 

One contextual problem with this identification is that the fall of angels isn't synchronized with the construction of the ark in either Scripture or 1 Enoch.

ii) Likewise, God's "patience" is in reference to Noah's disobedient neighbors. The ark was, itself, a sign of impending judgment. God gave human sinners time and opportunity to repent. 

iii) Angels are mentioned in v22, not because that ties into the netherworld setting of v19, but because that ties into the heavenly setting of the Ascension–and Session–of Christ. The Ascension not only represents the Son's "return" to heaven, but the Messiah's enthronement and coregency with the Father. All angels are subject to the Risen Lord. 

iii) But even if the passage refers to angels, that doesn't require an Enochic background. There's a similar motif in Isa 24:21-22:

21 On that day the Lord will punish    the host of heaven, in heaven,    and the kings of the earth, on the earth.22 They will be gathered together    as prisoners in a pit;they will be shut up in a prison,    and after many days they will be punished.

The "host of heaven" suggests angels. In context, fallen angels. They are "imprisoned," to await sentencing and final judgment. Cf. G. Smith, Isaiah 1-39 (B&H 2007), 424-25. 

4. After the Resurrection, Christ extended the opportunity of postmortem salvation to Noah's deceased contemporaries.

i) The passage doesn't actually say that. Rather than an offer of postmortem salvation, there's precedent for postmortem taunt-songs (e.g. Isa 14).

ii) Peter is exhorting his readers to remain steadfast in the faith despite persecution. It would subvert his message to hold out hope of a postmortem second chance. 

5. After the Resurrection, Christ proclaimed final condemnation to the damned.

That fits the context of Noah's disobedient neighbors, who spurned God's forbearance. That ill-fated generation constitutes a paradigmatic sample-group of the damned.