Observations / Questions:

v.1, "Therefore" points back to 3:18, "Christ suffered once for sins... he was put to death in the flesh..." and because verses 3:19-22 intervene, Peter reiterates the point here, "Christ suffered in the flesh..." and draws a new inference from it.

v.1 states, "...arm yourselves with the same way of thinking."  What way of thinking?

v. 18-22 present Christ as the epitome of a mindset presented in v.13-17, that "if you suffer for righteousness' sake, you will be blessed."

Consider Hebrews 12:2, "...looking to Jesus the founder and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God."

Christ 'armed' himself for the battle of suffering by setting his heart and mind on the joy that awaited him. Joy in the Father and joy in bringing us to the Father (3:18).

v.1-2, The purpose statement in v. 2 helps unravel the enigmatic phrase " whoever has suffered has ceased from sin."  Peter nowhere indicates an adherance to sinless perfectionism, he does however consistently exhort believers to real, practical holiness (1:13-19; 2:11-12). In the present context, it is the furnace of suffering that has a way of filtering one's priorities such that the pleasures of sin are shown to be short-lived and hollow, they cannot sustain and bear a soul through to the end; and so the sufferer turns away from sin in order to do God's will, counting on and sustained by his promises.

v.3, I love Peter's clarity: sin has had its day in your life, don't give it one more second.

v.4, Translating ξενίζονται as "they consider it strange" or "...foreign" seems to me to make better sense of the Ac-Res relationship than, "they are surprised," since the result of their perception is to malign.

The reality is, believers are "sojourners and exiles" (2:11) in this world.

v.5, Once again Peter points his readers to the last days to gain perspective and assurance (see 1:5,7,13; 2:12).  In this case, believers can be assured that those who malign their holy lifestyle will be held accountable. The aim is not to incite arrogance but to encourage his readers that men do not have the final say, God will bring all, living and dead to account.

v.6, Another tricky passage concerning which a lot of ink has been spilled. I'll only add a few reflections from the process of arcing this passage.

 Who are the 'dead'?  There is a clear identity that "those who are dead" are "judged in the flesh" and "live in the spirit".  The only biblically consistent conclusion I can see is that these are individuals who heard and believed the gospel and are now dead.

What judgment and what life do they experience?  The phrases "...in the flesh, ...in the spirit" have a parallel in 3:18 in which it is Christ who was "put to death in the flesh, but made alive in the spirit."  Also in 4:1, "Christ suffered in the flesh," and 4:2, the reference is to the rest of the believer's "time in the flesh."   In all cases, "in the flesh" refers directly to suffering in one's earthly life. In 3:18, "...in the spirit" is a clear reference to Christ's resurrection life.  There is no contextual reason to shift the referents in v.6. In other words, the aim of the gospel witness is to grant Christ's resurrection life even though there comes with the gospel the experience of hardships, suffering, maligning, even death.

What is the point?  Arcing highlights the importance of little words like γἀρ (for) - one of the most theologically significant words in the Bible. The question is, "what does v.6 support?" When I consider the summary of v.6 just stated, my answer is that it does not support v.4 or v.5 immediately, but the whole of the preceding arc (v.1-5) and especially the primary proposition of that arc which is, "...arm yourselves with the same way of thinking."  In this way, v.6 is thoroughly consistent with all that Peter has already written in order to edify and exhort these Christians struggling with the realities of suffering, namely: suffering is real but it is not the final word; death is not the end, the believer has a living hope and a certain future;  because the gospel does impart life.

Summary: The Christ-centered sufferer is made fit for battle by having a Christ-like mindset. This is a complete reordering of one's priorities giving no room for sin. The world around you will think this strange indeed and even bring curses down upon you, but God has the final say. Do not fear, even for those who have already died, the world sees another body in the grave, but we know better - they received the gospel... and they live!