Babylon a Code Name for Early Christians: Hippolytus (A.D. 170-236)

For he [the apostle John] sees, when in the isle Patmos, a revelation of awful mysteries, which he recounts freely, and makes known to others. Tell me, blessed John, apostle and disciple of the Lord, what didst thou see and hear concerning Babylon? Arise, and speak; for it sent thee also into banishment.

Treatise on Christ and antichrist, sec. 36.

(Roberts, Alexander, & Donaldson, James (Eds.), The Ante-Nicene Fathers: Translations of the writings of the fathers down to A.D. 325, Edinburgh: T & T Clark, Buffalo: The Christian Literature Company, 1886,  vol. 5, p. 211)