Revelation 12:1 (ESV)

1 And a great sign appeared in heaven: a woman clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars.

Signs in Scripture vouch for the validity and reliability of God’s words (see Judges 6:17; 1 Samuel 10:1–9; Matthew 16:1–4; John 2:23). At the blowing of the seventh trumpet, loud voices in heaven had declared a marvellous word concerning the Lord’s kingship (Revelation 11:15). Now God gives a sign to confirm to John and his readers the accuracy of that declaration.