The strong angel lifted his right hand to heaven and, in swearing by the Maker of heaven, earth, and sea,
did not take God's name in vain (Ex 20:7; Deut 5:11).
Making a solemn statement, the strong angel was affirming that when the Seventh Trumpet sounds, the mystery
of God is finished and no longer a mystery. Good news for God's prophets!
The mystery of God that the strong angel was referring to is revealed in the next chapter:
Then the seventh angel sounded, and there were loud voices in heaven, saying, "The
kingdom of the world has become the kingdom of our Lord and of His Christ, and He will reign forever and ever."
(Rev 11:15)
The mystery of God appears to be that the kingdom of the world becomes the Kingdom of God belonging to, not
only to God the Father, but to His Son as well! The New Testament alluded to this as well
(Mark 4:11; Col 2:1-2).