To be a beloved brother means that Onesimus is a Christian and must be treated as such. He is a fellow believer, a member in God’s church who has the same dignity and worth as any other church member. Even though he may be a slave in terms of his social status, he is a brother when it comes to his relationship with Philemon. Paul is not denying that Onesimus is still a slave,1 but he is calling for a transformation in the way that Philemon sees and relates to Onesimus.2
16 no longer as a bondservant but more than a bondservant, as a beloved brother especially to me, but how much more to you, both in the flesh and in the Lord.