The story of St. Onesimus is inextricable with the stories of Sts. Apphia and Philemon, and found in the Epistle to Philemon, which was written by Paul. I've heard the lists of the epistles that scholars are sure Paul wrote, those they aren't sure about, and those they are convinced he didn't write. The lists don't stick in my head, but this one seems like it must come from Paul. And since it is only twenty-five verses of one chapter, it's a good read for today if you have three to five minutes.
Find the BibleGateway link here.
In short, Onesimus was a runaway slave, the property of a Christian named Philemon. Having run from Colossus, Onesimus hid in Rome and eventually wound up finding shelter from Paul. Paul converted him to Christianity. The letter, carried by Onesimus to Philemon and his wife, Apphia, recommends welcoming the runaway slave home as a brother in Christ. Paul argues for Onesimus' freedom, and urges that he be admitted to the Church that meets in Philemon's house.
While it falls well short of an argument for universal emancipation, it must surely have been useful to abolitionists in the USA and elsewhere. Paul may not have been a radical, but the letter helps me think well of him. [Yes, I am sure he is relieved to have my good report this morning.]