Revelation 10:5-7

5 Then the angel I had seen standing on the sea and on the land raised his right hand to heaven. 6 And he swore by Him who lives for ever and ever, who created the heavens and all that is in them, the earth and all that is in it, and the sea and all that is in it, and said, “There will be no more delay! 7 But in the days when the seventh angel is about to sound his trumpet, the mystery of God will be accomplished, just as He announced to His servants the prophets.”

It is odd to see an angel swearing an oath or promise to God. He announces in the most powerful way imaginable to the most powerful force existing that there will be no more delay. This is in answer to the cry of the Martyrs back in 6:10 upon the breaking of the 5th seal. When this last trumpet is sounded the fulfillment of establishing God’s restoration plan for the world and man as it was intended will be done.

What is the mystery of God? God revealed His mystery to the Church through Paul:

Colossians 1:24 - I now rejoice in my sufferings for you, and fill up in my flesh what is lacking in the afflictions of Christ, for the sake of His body, which is the church, of which I became a minister according to the stewardship from God which was given to me for you, to fulfill the word of God, the mystery which has been hidden from ages and from generations, but now has been revealed to His saints. To them God willed to make known what are the riches of the glory of this mystery among the Gentiles: which is Christ in you, the hope of glory.
The following Old Testament passages show God's mystery declared by the prophets Moses and Isaiah:

Deuteronomy 32:43 - "Rejoice, O Gentiles, with His people; for He will avenge the blood of His servants, and render vengeance to His adversaries; He will provide atonement for His land and His people."

Isaiah 11:10 - "And in that day there shall be a Root of Jesse, who shall stand as a banner to the people; for the Gentiles shall seek Him, and His resting place shall be glorious."

Isaiah 42:1-6 - "Behold! My Servant whom I uphold, My Elect One in whom My soul delights! I have put My Spirit upon Him; He will bring forth justice to the Gentiles. He will not cry out, nor raise His voice, nor cause His voice to be heard in the street. A bruised reed He will not break, and smoking flax He will not quench; He will bring forth justice for truth. He will not fail nor be discouraged, till He has established justice in the earth; and the coastlands shall wait for His law." Thus says God the LORD, who created the heavens and stretched them out, who spread forth the earth and that which comes from it, who gives breath to the people on it, and spirit to those who walk on it: "I, the LORD, have called You in righteousness, and will hold Your hand; I will keep You and give You as a covenant to the people, as a light to the Gentiles...."

Isaiah 49:6-10 - "Indeed He says, 'It is too small a thing that You should be My Servant to raise up the tribes of Jacob, and to restore the preserved ones of Israel; I will also give You as a light to the Gentiles, that You should be My salvation to the ends of the earth.' "

Do you remember the story from Jesus about the wedding or great banquet in Mark 22 or Luke 14? Here is Luke 14:16-23:

16 Jesus replied: “A certain man was preparing a great banquet and invited many guests. 17 At the time of the banquet he sent his servant to tell those who had been invited, ‘Come, for everything is now ready.’

18 “But they all alike began to make excuses. The first said, ‘I have just bought a field, and I must go and see it. Please excuse me.’

19 “Another said, ‘I have just bought five yoke of oxen, and I’m on my way to try them out. Please excuse me.’

20 “Still another said, ‘I just got married, so I can’t come.’

21 “The servant came back and reported this to his master. Then the owner of the house became angry and ordered his servant, ‘Go out quickly into the streets and alleys of the town and bring in the poor, the crippled, the blind and the lame.’

22 “‘Sir,’ the servant said, ‘what you ordered has been done, but there is still room.’

23 “Then the master told his servant, ‘Go out to the roads and country lanes and compel them to come in, so that my house will be full.”

God plans to reconcile the Jews and the Gentiles under the Messiah in an earthly kingdom. Whosoever, that is Jew or Gentile, who acknowledges that there is One true living God and His Son is Jesus their Savior will walk with Him!

We should be reminded of the Words of Jesus in John 12:27-32:

27 “Now my soul is troubled, and what shall I say? ‘Father, save Me from this hour’? No, it was for this very reason I came to this hour. 28 Father, glorify Your name!”

Then a voice came from heaven, “I have glorified it, and will glorify it again.” 29 The crowd that was there and heard it said it had thundered; others said an angel had spoken to Him.

30 Jesus said, “This voice was for your benefit, not mine. 31 Now is the time for judgment on this world; now the prince of this world will be driven out. 32 And I, when I am lifted up from the earth, will draw all people to myself.”

The voice that spoke as thunder to Jesus is most likely the same that is speaking to John here. We should also keep this declaration of what is to happen mindful as we continue to read in Revelation.