Possible meanings are "authority because of Christ" or "courage because of Christ." This can also be translated as "authority because I am an apostle of Christ."
Possible meanings are 1) "because I know that you love God's people" (UDB), 2) "because you love me," or 3) "because I love you."
## being such a one as Paul the old man, and now also a prisoner because of Christ Jesus ##
These are reasons for Philemon to do what Paul was about to ask him to do. It can also be translated as "because I am Paul, I am an old man, and I am now a prisoner because of Christ Jesus."