Revelation 17:6

She had drunk the blood of believers, the saints generally and witnesses to Jesus Christ specifically (cf. Revelation 11:10; Revelation 13:7; Revelation 13:15). This system had destroyed true believers and rejoiced in their deaths. This revelation amazed John. A system purporting to honor God was killing His faithful followers! Alternatively John may have wondered why God allowed her to live, or because he did not understand the meaning of what he saw, or because he saw a splendidly attired woman instead of a ruined city. In his day the Roman Empire was the greatest manifestation of Babylonianism.