While Paul was in prison he was helping a young believer named Onesimus grow in godliness.  Onesimus would leave the prison soon so Paul wrote to Philemon to help the young believer once he was free. Philemon was faithful to God and showed great kindness to others.  Paul trusted Philemon with Onesimus.

Philemon 1:

4 I always thank my God when I pray for you, Philemon, 5 because I keep hearing about your faith in the Lord Jesus and your love for all of God’s people. 6 And I am praying that you will put into action the generosity that comes from your faith as you understand and experience all the good things we have in Christ. 7 Your love has given me much joy and comfort, my brother, for your kindness has often refreshed the hearts of God’s people.
8 That is why I am boldly asking a favor of you. I could demand it in the name of Christ because it is the right thing for you to do. 9 But because of our love, I prefer simply to ask you. Consider this as a request from me—Paul, an old man and now also a prisoner for the sake of Christ Jesus.

10 I appeal to you to show kindness to my child, Onesimus. I became his father in the faith while here in prison. 11 Onesimus hasn’t been of much use to you in the past, but now he is very useful to both of us. 12 I am sending him back to you, and with him comes my own heart.

13 I wanted to keep him here with me while I am in these chains for preaching the Good News, and he would have helped me on your behalf. 14 But I didn’t want to do anything without your consent. I wanted you to help because you were willing, not because you were forced.