For this reason I, Paul, a prisoner for Christ Jesus on behalf of you Gentiles
Ephesians 3:1 ESV
The contrast could not be more jarring. Unlike Paul’s salutation at the beginning of the letter, where he referred to himself as an apostle of Christ by the will of God, now it’s a prisoner for Christ on behalf of the Gentiles. The blessings he’s been describing seem to have turned into a bitter curse.
Paul is not unaware of this contrast and it seems it was too much for him to pass over without explanation. Therefore he leaves off his train of thought to explain to his readers how his imprisonment does not negate the great blessings of Christ, but is rather the very instrument by which Christ has chosen to bring his blessings to the Gentiles.
Paul writes the next 12 verses to explain how his sufferings are bringing about the salvation of the Gentiles. His imprisonment does not contradict what he has been writing about our blessings in Christ, it brings about the blessings for those under his care. Therefore he can say in verse 13,
So I ask you not to lose heart over what I am suffering for you, which is your glory.
Ephesians 3:13 ESV
Now that Paul has established the purpose of his sufferings in an age of blessings, Paul can get back to his original train of thought, a prayer for strength on behalf of his readers,
For this reason I bow my knees before the Father
Ephesians 3:14 ESV
Note: a prayer for stength would not make that much sense in a uptopian world. Now that hardships and sufferings are acknoledged, we see our desperate need of God assistance.
But now to the main point, having established the blessings in Christ and the reality of suffering, in Ephesians 4:1, Paul uses the suffering in his own ministry as a motivation tool for the Gentiles.
I therefore, a prisoner for the Lord, urge you to walk in a manner worthy of the calling to which you have been called
Ephesians 4:1 ESV
In this one transitional sentence, Paul is moving from doctrine to duty. By referring to his imprisonment again, it seems to me that Paul is saying, “Therefore, just as I have walked in a manner worthy of my calling by pouring my life out for your sake, even to the point of imprisonment (along with all my other beatings, stonings, etc.), you walk in a manner worthy of your calling.” It is this worthy manner which Paul will unpack for the rest of the letter. Basically he is saying, don’t let my suffering be for naught. I've done my part in your salvation, now you do yours!
One last observation, our calling is to lay down our lives and to suffer for the sake of others just as Paul did. In fact, this is what he will explicitly call husbands to do in chapter 5.