Albert Barnes Commentary Revelation 17:7

"And the angel said unto me, Wherefore didst thou wonder? I will tell thee the mystery of the woman, and of the beast that carrieth her, which hath the seven heads and the ten horns." — Revelation 17:7 (ASV)

And the angel said unto me, Wherefore didst thou marvel? The angel was undoubtedly struck by John's appearance as he stood transfixed in astonishment. The question asking him why he wondered was intended to show him that the reason for his surprise would be removed or diminished, because the angel would proceed to explain the vision so that John might gain a correct understanding of its design.

I will tell thee the mystery of the woman. Regarding the word mystery, see the comments on Revelation 17:5.

The meaning is, "I will explain what is meant by the symbol—the hidden meaning that is contained within it." That is, the angel would explain it to such an extent that a proper understanding of its significance might be obtained. The explanation follows in Revelation 17:8-18.

And of the beast that carrieth her, and so on (Revelation 17:3 describes this beast).