|St. Onesimus, the Saved Slave|
The book of Philemon which contains only twenty-five verses in total was written by Paul probably around 59-61 AD during his Roman imprisonment. The letter is addressed to a certain Christian named Philemon (v. 1), the owner of a slave Onesimus (v. 10-12), and (if tradition serves us right) a member of the church at Colossae. Onesimus apparently ran away from his master (Philemon), a crime punishable by death during those days, and ended up encountering Paul in Rome whereby he was converted to the Christian faith. In Philemon 10 we see how deep Paul has cultivated his personal relationship with the run-away slave, but Paul seems to have considered it unethical to continue making use of Onesimus' services since the slave was still, technically, owned by Philemon. So he eventually decided to send the slave back to his master. In verses 12-14 we read thus:
"I am sending him back to you, sending my very heart. I would have been glad to keep him with me, in order that he might serve me on your behalf during my imprisonment for the gospel, but I preferred to do nothing without your consent in order that your goodness might not be by compulsion but of your own accord." (Phm. 1:12-14)Paul knew death was the normal punishment for run-away slaves, so he entreated Philemon to receive Onesimus "no longer as a bondservant but more than a bondservant, as a beloved brother" (v. 16). But, humanly speaking, why would anyone repay a run-away slave with such favorable treatment? On what ground does Onesimus deserve this warm welcome from his master? Observe how Paul elicited Philemon's kindness towards Onesimus:
"So if you consider me your partner, receive him as you would receive me. If he has wronged you at all, or owes you anything, charge that to my account. I, Paul, write this with my own hand: I will repay it—to say nothing of your owing me even your own self." (Phm. 1:17-19)
Onesimus has merited death by fleeing from his master Philemon, but Paul stood in the way and asked Philemon to receive the slave as he would receive him, echoing the imputation of Christ's righteousness on our account. The apostle also told Philemon that if ever Onesimus has wronged him or owe him anything, he himself will pay for it, echoing the imputation of our sins on Christ's account. Here we see how central the doctrine of justification was in Paul's life, even in his dealing with social cases like this.
This clear allusion to the doctrine of double-imputation that we find in the book of Philemon, I believe, is why God providentially included this document in the canon of Scriptures. I don't know of any ancient fathers of the church who saw things the same way, but beyond this perspective it would be hard to look for any good reason why this book should be included in the canon (other than that it was penned by an apostle).