Philemon: Blood Brother

Now listen, before you start grumbling about the woodenness of this translation, what I want you to see is what I finally—after having read this letter many, many times, studied it, went through commentary after commentary, for a good forty years—finally saw tonight.

Philemon: An Introduction

So, Paul wrote a short, personal note to Philemon as a cover letter to accompany the open epistle for the Colossian churches, and to discuss their mutual acquaintance, Onesimus.