What does Revelation 17:7 mean?

The angel speaking here is the same one who brought John into a vision about a "great prostitute" (Revelation 17:1–6). This symbolic woman is the religious aspect of Babylon. At the end of that description, John said he "marveled greatly," meaning he was amazed or astonished. The attending angel said he would explain the mystery of the woman and of the beast with seven heads and ten horns that supported her. Apparently, the angel saw the look of bafflement on John's face and promised to reveal the meaning of what he was seeing.

Although John was an apostle, he did not know everything. He often depended upon a revelation from heaven to clarify matters. We, too, will never outgrow our need to depend upon God for enlightenment. Fortunately, God has given His Holy Spirit to us to help us understand what He has revealed. First Corinthians 2:9–12 explains, "As it is written, 'What no eye has seen, nor ear heard, nor the heart of man imagined, what God has prepared for those who love him'—these things God has revealed to us through the Spirit. For the Spirit searches everything, even the depths of God. For who knows a person's thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God. Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God."