a prisoner of Christ Jesus

I am listening to John MacArthur's messages on Acts and in particular Acts 21:26 and on to the end of Acts.
Paul, from Acts 21:26 on is a prisoner.
In Ephesians 3:1 he says "For this reason I Paul, a prisoner of Christ Jesus for you Gentiles---".
In Philemon 1:1 "Paul, a prisoner of Christ Jesus,...".
In Philemon 1:9 "--being such a one as Paul, the aged, and now also a prisoner of Jesus Christ."

I have recently read again for the ??? time, Pilgrim's Progress, written by John Bunyan, for the most part while he was in prison more or less continuously from 1660 to 1672.
He too could say that he was a "prisoner of Christ Jesus"; and say with Paul in 1 Corinthians 9:16 "For if I preach the Gospel, I have nothing to boast of, for necessity is laid upon me; yes, woe is me if I do not  preach the Gospel!"

And finally, we see that the persecution of believers who proclaim and confess Christ still become "prisoner[s] of Christ Jesus..." today.  There are many, one of whom is Youcef Nadharkhani, a Christian minister in Iran who has been in prison for just under 3 years for 'apostasy' from Islam.