Saint Peter was the first bishop of Rome. As such he's considered the founder of the papacy, and, as one of the 12 apostles, the major link in the apostlic succession for the Catholic church. He's all over the gospels and Acts, and there are many other traditions about him. I, of course, am only going to talk about his martyrdom.
After the Great Fire of Rome, Nero blamed the Christians and celebrated his regnal anniversary with a great crucifiction of them, so Peter's death is dated to 13 October 64 AD. Apparently Peter said he wasn't worthy of being crucified in the same way as Christ, so the Romans crucified him upside down.
The Crucifiction of Saint Peter
So anyway, one symbol of Saint Peter, sometimes worn by people who wish to show their allegiance but don't believe that they're worthy of wearing a symbol of Christ, is an inverted cross. So for this costume, I reckon a nice, large, prominent inverted cross. It doesn't get any more Christian than that.
 Ignoring, as is traditional, Simon Magus.
 The Eastern and Oriental Orthodox churches also recognise apostlic succession, but also have more apostles to choose from amongst their founding bishops.