Then the angel, the one I saw standing upon the sea and the land, lifted up his right hand to heaven and swore to the One living into the ages of the ages, He who created the heaven and the things in it, and the land and the things in it, and the sea and the things in it—time would no longer be [extended]:
But yet in the days of the voice of the mighty angel, whenever he is about to trumpet, then the mystery of God was completed as announced to His own servants, the prophets.
Revelation 10:5-7
Imagining this scene in my mind’s eye, I see the angel lifting his right hand up to heaven (and I think the little scroll had to have been in his left hand), and vowing that the time had come for God’s mystery to be completed, just as God had announced to the prophets of old. When the seventh trumpet is sounded, the whole thing will have been revealed.
What a way to rivet our attention!

Historicist
If Revelation includes the scope of the past two thousand years, then this passage speaks to a time that set the course of Protestantism.
Just as God’s people the Jews had been given a thousand years and more to repent, so also had the Church been given the same grace. That is to say, the Medieval Church which had become encumbered with corruption, apostacy, and false doctrine. Tragically, just as the ancient Jewish leaders had killed the prophets God had sent to them – the Jews who killed both the Lord Jesus and the prophets – so also did the Medieval Church kill tens of thousands of dissenters and reformers who were seeking a purer expression of faith.
In the first century, when Jesus came as Messiah, this was the “seventh trumpet,” so to speak, for the Jewish people. If they rejected their Messiah, then Jerusalem and Judea would be no more.
Now, the angel was announcing the seventh trumpet ultimatum for the church. There would now be no more delay. God’s judgment had arrived, in the form of the Reformation.
Some historicist scholars point out that Jesus came to the Jewish people about fifteen hundred years after Israel had become a nation. Now, the Reformation had come fifteen hundred years after the founding of the Church.

Preterist
Focusing in on Jerusalem’s fall, preterist expositors remind us about the martyrs swirling in the blood drain of God’s heavenly altar.
Then, when he opened the fifth seal, I beheld beneath the altar the souls of the ones having been slain because of the word of God and because of the testimony they were holding.
And they cried out in a great voice, saying, “Until how long, Lord, the Holy and Trustworthy, True One, are you not judging and avenging our blood out of the ones living upon the earth?”
Revelation 6:9-10
Remember, they were given white robes and an invitation to rest, for much was to happen until what they were asking for could be given, including many more who would die for Jesus.
Now, the angel is announcing, that rest period has come to an end, the prayers of the martyrs, the prayers of all the persecuted Christians on earth which have been translated to heaven and are now part of the altar of incense before God, shall be answered.
God’s judgment against the first subjugators of Christians (the Jewish religious leaders), in the form of the Roman army, would be devastating and final.
God’s mystery would be made complete in that the center focus of the church would shift from the predominantly Jewish mother church in Jerusalem, to Gentile churches.
This is the reason that I, Paul, am a prisoner for Christ Jesus for the sake of you gentiles, for surely you have already heard of the commission of God’s grace that was given me for you and how the mystery was made known to me by revelation, as I wrote above in a few words, a reading of which will enable you to perceive my understanding of the mystery of Christ. In former generations this mystery was not made known to humankind, as it has now been revealed to his holy apostles and prophets by the Spirit:
that is, the gentiles have become fellow heirs, members of the same body, and sharers in the promise in Christ Jesus through the gospel.
Ephesians 3:1-6 (NRSV)
Paul wrote regularly of this mystery, in which God would join together Jews and Gentiles into the one Body of Christ. Because Jerusalem, the Sanhedrin, and the Temple all stood in opposition to this mystery, God would remove them in judgement.
Futurist
Commentators who interpret Revelation’s prophecies as still future to us today, see a different mystery of God coming to completion—and it is a mystery most of us have wondered over ourselves.
The mystery of evil.
Why does Almighty God suffer evil to exist? More pointedly, why the serpent in the Garden? Why is God’s ancient enemy permitted so much freedom to ruin God’s beautiful creation? What possible good can come from all the wickedness, corruption, suffering, and death that Satan, the fallen angels, and the forces of darkness engender?
But, at some point, at the sounding of the seventh trumpet, this strange mystery will come to its finish and perhaps the answers revealed. All the trauma and tragedy will be brought to an end under God’s wise and just judgment, and the triumph of evil will be taken away.
But not all futurist theologians think this is about the mystery of evil. Some see the mystery of God is about God, about the kingdom of God being established on earth. This is a broader category, and possibly encompasses all the disputed doctrines in Christendom—about the struggle between good and evil, about predestination held against humankind exercising free will, about the mystery of God’s just judgment and the great power of God’s grace and mercy happening all at once in one event.
How can it be? It is a mystery that will come to completion and be revealed in all its astonishing array of light.

Spiritual
Theologians who take the spiritual view agree with the preterist understanding of the mystery of God. Throughout Paul’s letters, he speaks of what would otherwise have seemed not only unnatural but impossible. God would take the mature and rich olive tree of Israel, that God had been tending in God’s garden for thousands of years, and make cuts in it. Into those cuts would go clippings from wild, untamed olive trees of the Gentiles. (Paul describes this disturbing mystery in Romans 11.)
It is the opposite of what any arborist or orchard tender would undertake.
But that is the mystery.
Now to God who is able to strengthen you according to my gospel and the proclamation of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages but is now disclosed and through the prophetic writings is made known to all the gentiles, according to the command of the eternal God, to bring about the obedience of faith—to the only wise God, through Jesus Christ, to whom be the glory forever! Amen
Romans 16:25-27 (NRSV)
Paul wrote to the church in Colossae, both Jewish and Gentile believers,
I want their hearts to be encouraged and united in love, so that they may have all the riches of assured understanding and have the knowledge of God’s mystery, that is, Christ …
Colossians 2:2 (NRSV)
The Gentile Christians were to be enriched by the scriptures, wisdom, godly knowledge and reverence of the Jewish people.
Still, when the angel announces this mystery will be made complete at the seventh trumpet, it is by way of declaring God’s judgment of justice and mercy.

The four perspectives taken from Revelation: Four Views A Parallel Commentary, edited by Steve Gregg