John records Jesus’ ringing “It is finished” in his crucifixion narrative. But when he gets to Revelation, John records another declaration that things are “finished.” The angel who gives John the book to eat swears that “the mystery of God is finished” (Revelation 10:7).
In context, the mystery of God has something to do with the renewal of creation, since the angel swears in the name of the Creator (v. 6). It’s also something that is about to happen, as the oath is specifically that “there shall be delay no longer” (v. 6).
Jesus has died; Jesus has risen; but there is yet something to complete. The mysterious birth of the new creation is not yet finished, though John is assured that it will be completed soon. And it is completed within the scope of Revelation, as the Bridal city descends from heaven.
The link of Revelation 10 with the crucifixion is not fortuitous. Jesus finishes His work in His death; and the mystery of God is completed when martyrs are slaughtered, joined to the death of Jesus. Jesus blood and death completes His work, but that completion is realized in another act of completion, in the filling up of the sufferings of Christ in the suffering of His saints.