'In Christ' in the Bible

not as a slave [only], but more than a slave, as a dearly loved brother. [He is that] to me especially, but how much more to you, both in a physical way [i.e., by continuing to serve as your slave] and in [the fellowship of] the Lord [i.e., now also as your brother in Christ].

So, brother, I would like to have your help in [the fellowship of] the Lord; cheer me up in Christ [i.e., by welcoming Onesimus back on good terms].