Even if all [others], not I.
et si omnes scandalizati fuerint sed non ego (Mk 14:29).
et si omnes scandalizati fuerint in te ego numquam scandalizabor (Mt 26:33).
"The crucial scene in Fest's childhood memoir is his father's lesson to his two eldest sons. He told them to write down a Latin motto from the Gospel according to Saint Matthew, and to commit it to memory: 'Etiam si omnes—ego non!'—Even if all others . . . not I! Fest does not mention this, but a variant of these words was written on the house of Philipp von Boeselager, one of the officers who plotted to kill Hitler in July 1944. He, too, was a pious Catholic."
Ian Buruma, "A very superior German liberal," reviewing Not I: memoirs of a German childhood, by Joachim Fest. New York review of books 61, no. 13 (August 14, 2014): 68 (67-68).
Contra Buruma and others, it is closer to Mark than Matthew.
It's a great motto, and one to which Johannes Fest (!) and Philipp von Boeselager did live up, but Buruma gives no indication that either he or anyone else noticed the irony. It's a fantastic motto, but one that (given both the Lord's response and Peter's subsequent capitulation) I would be afraid to make my own.