Babylon, a Code Name for Early Christians: Eusebius (A.D. 345-20 September 420)

And Peter makes mention of Mark in his first epistle which they say that he wrote in Rome itself, as is indicated by him, when he calls the city, by a figure, Babylon, as he does in the following words: “The church
that is at Babylon, elected together with you, saluteth you; and so doth Marcus my son.”

The church history of Eusebius, bk. 2, ch. 15

(Schaff, Philip, & Wace, Henry (Eds.), A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Edinburgh: T & T Clark, New York: Charles Scribner's Sons, 1904, vol. 1, p. 116)