8 Though Paul had apostolic authority over Philemon, Onesimus and the church in Colossae (see note on Col. 1:1), he appealed to Philemon on a more foundational basis—God’s love in the gospel and the love it fosters between fellow believers.
Paul's long service of love - Philemon 1:9
9 an old man . . . a prisoner. Paul appeals to Philemon also on the basis of his long service of love to God’s people, even to the point of imprisonment (v1, 9).