Revelation 17:5 - “The name written on her forehead was a mystery: Babylon the great, the mother of prostitutes and of the abominations of the earth.”
'She' is the mother of the prostitutes, i.e. the worst of the world's religious entities - Christendom.
Revelation
18:4 - “Then I heard another voice from heaven say: “ ‘Come
out of her, my people,’ so that you will not
share in her sins, so that you will not receive any of her plagues.”
God's people are Christians - believers in His son. Christendom has/is perverting & corrupting Christianity.
1 Peter
4:17 - “For it is time for judgment to begin at the household
of God.” (Hebrews 3:6)
Revelation chapters 2 & 3 elaborate.