New Defender's Study Bible Notes
5:13 Babylon. Babylon had a large Jewish population, and Peter had evidently gone there to evangelize and make disciples among them, since his special calling was to the Jews, as Paul’s had been to the Gentiles (Galatians 2:7). Some have speculated that Babylon was a mystical name for Rome, but no basis exists for this idea, and there was no indication that Peter had ever been there. Paul wrote a letter to Rome about this same time and had no hesitancy in calling the city by name (Romans 1:7).
5:13 Marcus. John Mark was Peter’s son “in the faith,” and evidently received much of the information for his gospel from Peter.