Understand that, in Revelation 18:4, God is not calling on the world to repent from their sins and convert to Christian (Messianic) Judaism. 'My people' in the verse implies that they already believe in Jesus God except that they are at the wrong place.
What do those who believe in Jesus God do?
Those who believe in Jesus go to church or to synagogue. Therefore, if they are called upon to come out, it implies necessarily that the churches, the synagogues are Babylon which the serious believers in Jesus must come out of.
Hence, Babylon is Christianity and Protestantism.