Can we say for sure that Onesimus stole from his master?

Answer:

Paul tells us in the book of Philemon that Onesimus was a slave (Philemon 16) who became a Christian after meeting Paul while Paul was in prison in Rome (Philemon 10). Paul makes it clear that Onesimus was not a good worker (Philemon 11) and that he left without the consent of his master (Philemon 14). In other words, Onesimus was a runaway slave.

Paul did not say Onesimus had stolen anything. "But if he has wronged you or owes anything, put that on my account" (Philemon 18). The "if" indicates that Paul is covering contingencies. He did not know what harm Onesimus' running away might have caused Philemon or what expenses Philemon might have incurred in trying to recover his lost slave. It was Paul's desire that Philemon not hold Onesimus accountable. Even though Paul offers to pay any damages or expenses on Onesimus's behalf, he makes it clear that he expects to find no payment necessary. "I, Paul, am writing with my own hand. I will repay--not to mention to you that you owe me even your own self besides" (Philemon 19). As Adam Clarke states, "Had the apostle been assured that Onesimus had robbed his master, he certainly would not have spoken in this hypothetical way."

Almost every commentary I read mentions that a few suppose Onesimus might have stolen something, but all agree that it is not a necessary inference and there is no reason to make the conclusion.