Timothy is called our brother
as an acknowledgement that he is a fellow believer together with Paul and Philemon.1 Right from the beginning of his letter, Paul uses familial language and applies it in a way that shows how we ought to think of one another in God’s church. This is of course important for what he wants to address in this letter.
1 Paul, a prisoner for Christ Jesus, and Timothy our brother,To Philemon our beloved fellow worker