Revelation 10:7

But in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished, as he hath declared to his servants the prophets.

Cross-reference

Revelation 11:15-18 records the seventh angel sounding and God's reign beginning — directly fulfilling the promise in 10:7.

Revelation 16:17 records the seventh bowl declaring 'It is done!' — echoing the completion theme of the seventh trumpet.

Daniel 12:7 shows an angel swearing that all things will be finished after a set time — directly parallels the angel's oath and fulfillment in Revelation 10:5-7.

Romans 16:25 reveals the mystery of Christ kept secret — the same mystery that Revelation 10:7 declares is finished at the last trumpet.

Ephesians 3:3-9 explains the mystery of Christ including Gentiles — the same 'mystery of God' that is completed in Revelation 10:7.