Possibly the most difficult passage.....

......in the New Testament!
Seriously!
Are ya ready?

For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the Spirit. 19 After being made alive, he went and made proclamation to the imprisoned spirits— 20 to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. In it only a few people, eight in all,were saved through water, 21 and this water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a clear conscience toward God. It saves you by the resurrection of Jesus Christ, 22 who has gone into heaven and is at God’s right hand—with angels, authorities and powers in submission to him. (I Peter 3:18-22)

Wow! This is a mine field!
It seems to me that Peter is trying to explain the point that he made in the previous verses....that we can trust God to vindicate us as we witness in a hostile world. He uses Christ as his main example, showing that His unjust suffering resulted in a witness seen by many, and that He was vindicated through His resurrection and His ascension to the right hand of God.
Peter also gives his readers an "example within an example," because he also mentions Noah, who was vindicated by God. So, his readers should be encouraged, knowing that God will vindicate them.

OK. No worries there.
Easy peasy, right? I mean, we can think of other examples, too, of peeps who were bold and who suffered, and who were vindicated.
Why did he make it so convoluted? (Grin)
And by the way, we are not the only ones who have to read and re-read this. Most commentators will acknowledge that these are some of the most difficult verses in the New Testament to interpret. Martin Luther, way back in the 1500's, wrote that this is perhaps the most obscure passage in the New Testament. I read many commentaries that were honest and said that the meaning of each word in a couple of these verses varies wildly -- and one that said each of the words in verse nineteen has been differently understood at different times! (Grin)

Anybody besides me kinda want to skip past?
Nooooooo, not really!
There may be differences of opinion on the details, but the overall point is clear.....since Jesus was vindicated, we, too, can bear witness through suffering and trust God to vindicate us.

We're going to stick to the core principles here, and make a quick side trip into the difficult portions. I don't want to get caught up in arguments, just stick with the things that are important.
Sound good?

Let's dive in!
The very first word of verse 18 is "for," kinda like saying "because." Peter is explaining what came before; namely that we might suffer for doing what is right.  He's pointing us to Christ as our chief example, but then he goes beyond the example to Christ's death as a substitute for us.
I think the overall meaning here is this: Peter is urging us to imitate Christ, but then he wants us to see there is a point at which the imitation stops, and we bow before our Lord, Who is glorified over all other beings.

Christ suffered as "the just for the unjust." Or, to put it another way, the righteous One suffered for the unrighteous. Only Christ is righteous.....how many of us, in the midst of suffering or persecution or humiliation, think to ourselves, "I don't deserve this!"  We are kinda like that guy in the temple that looked over at the less wealthy, less well-dressed man who was abjectly humble, and regretful for his sins. The first guy looked down his nose and said "Wow, I'm so glad I'm not like him!" (Yeah, I know, that IS a paraphrase of the verses. You can check out the real story in the eighteenth chapter of Luke!)
Seriously, though, aren't we being like that Pharisee, when we look around us and think, hey, I'm a good person! I mean, look at me! I don't do drugs or cheat on my spouse. I'm honest (most of the time) and I'm law-abiding (except when I think there's no highway patrols around), and I don't kill or steal...... why should I suffer when terrible sinners get away with murder and have a great life and prosper?
Oy vey.
I'm here to tell ya -- we are comparing ourselves with the wrong standard. If we compare ourselves with the absolute righteousness of Jesus Christ, we would realize that the only thing we deserve is hell. Sorry, but that's the facts, ma'am, as Sergeant Friday would say.  Each of us has broken the rules over and over, even as believers. We put other gods before our Father, and we allow things to become idols in our lives. We take His name in vain, and don't keep His day holy. We certainly don't love the Lord our God with all our heart, and love our neighbors after that..... If we think we are not in this basket, we need to read the Sermon on the Mount. Jesus showed everyone what the true standard of godly living was.

We have noted before that God, in His perfect justice, can't just shrug off our sins. He took our sins and put them on Jesus, to bear the penalty that we deserve. Why? In verse 18, Peter says it was to "bring us to God." That word in the Greek carries with it the meaning of an admission to an audience with a mighty king. THE mighty King.  Back then, and now, too, I guess, you didn't just stroll into the presence of a potentate or monarch and say, "Howdy!" 
You had to have someone to introduce you properly. Because the righteous One, Jesus, bore our sins, He can bring us into the presence of the Great King.

Here's another awesome fact: Christ's death for sins was "once for all." His death was sufficient to pay for all the sins we have committed and those we will commit. Paul makes this point repeatedly in the letter to the Hebrews, when he contrasts the repeated sacrifices of animals (the old covenant) with the once for all sacrifice of Jesus, in the new covenant.

The point is this (and this brings tears to my eyes, and may to yours, as well) if we have put our trust in Christ, then our sins are on Him. We've been reconciled to God once, and for all. God wants all of us to come to the understanding that His acceptance of us is not based on our performance -- it's His grace..... Christ died for our sins.
One of my favorite hymns, and one that I sang for a relative's funeral, is "It is Well." Here are some lines from that song:

My sin, Oh, the bliss of this glorious thought,
My sin, not in part, but the whole.
Is nailed to the cross and I bear it no more,
Praise the Lord, praise the Lord, Oh my soul!

Amen!
We'll get further into the weeds, tomorrow..... (Grin)