John Gill Commentary Philemon 1:21

"Having confidence in thine obedience I write unto thee, knowing that thou wilt do even beyond what I say." — Philemon 1:21 (ASV)

Having confidence in your obedience
In his obedience of faith to Christ, and his Gospel; he having been made willing in the day of his power to serve him, as well as to be saved by him; and being constrained by his love, and the Spirit of Christ having wrought in him both to will and to do of his good pleasure:

I wrote to you, knowing that you will also do more than I say ;
the knowledge the apostle had of Philemon's cheerful obedience to Christ in all the parts of duty, encouraged him to write to him, on this head; believing that he would even do more than he had desired of him.