Literally, John tells us that when the seventh angel was “about to sound” the mystery of God would be finished, just as the Lord had claimed through his servants, the prophets. So, these things will be fulfilled just before the sounding of the seventh trumpet, according to John. Now, there are several mysteries of God, such as the mystery of godliness (1Timothy 3:16, the mystery of blindness of the Jews (Romans 11:25), the mystery of the faith (1Timothy 3:9), the mystery of iniquity (2Thessalonians 2:7) etc., but all were preached through the Gospel. These mysteries and therefore the goal of the Gospel would be completed by the time the seventh angel was ready to sound his trumpet.
The word declared (G2097 – euaggelizo) is usually used for preaching the Gospel. (Matthew 11:5; Luke 3:18; 4:18; Acts 5:42; Galatians 1:8-9; 3:8 etc.). So, by the mighty Angel saying: “…the mystery of God should be finished…” (Revelation 10:7; cf. 1Corinthians 2:1-2, 7; Romans 16:25-26), he means that the commission given the Apostles to preach the Gospel and make disciples of all nations (Matthew 28:19-20) should be finished. Notice, when the two witness have finished their testimony (Revelation 11:7), the seventh angel is about to sound (Revelation 11:15). Therefore, it seems that the mystery of God is finished when the ministries of the two witnesses are finished.
Evidently, therefore, these things have to do with the Jewish state (1Peter1:12). That is, Peter claimed that the prophets of the Old Covenant wrote of the days of Jesus and the Apostles in the first century AD! There is absolutely nothing in Scripture to indicate they spoke of a day 2000 years into Peter’s future. Peter claimed these same prophets searched diligently seeking to understand the time that the Spirit of Christ (which was in them) indicated, but instead they were told their own words were sealed and wouldn’t be opened until the day appointed (1Peter1:10-12).
Keep in mind that although the Lord did, indeed, reveal his plans to the prophets of the Old Covenant, those plans were not clear, nor were they understood by the prophets the Lord used to publish them. Nevertheless, in the New Covenant the Lord, himself, came (Hebrews 1:1-2) and gave his servants, the writers of the New Covenant Scriptures, a surer word of prophecy (1Peter 1:19), and we are encouraged by Peter to take heed of it. Therefore, we need to interpret the Old Covenant prophets in the context of the sayings of Jesus and the Apostles, not the other way around. The prophets declared that the gentiles would come into the Kingdom of God (Isaiah 60:3), during the days of the kings of the fourth worldwide kingdom (Daniel 2:44), and in that day the LORD would be King over all the earth (Zechariah 14:9; cf. Revelation 11:15-19).
All prophecies beyond the first century therefore, that claimed the Lord was soon to come have failed. The Apostles pointed to a time in their near future, when Jesus would come. Yet, most modern scholars would claim that these very people, whom Jesus chose to represent him, were wrong in presuming Jesus’ soon return in the first century AD. Nevertheless, each and every wanabe prophet, since the time of the Apostles, were wrong. How many people need to be proved wrong before scholars will reconsider the words of the Apostles that Jesus returned, when he said he would return (Matthew 16:27-28), i.e. in the very generation of Jesus’ own listeners?
The Lord promises never to do anything without informing his elect (cf. Amos 3:7-8; Jeremiah 7:25; 25:4), but didn’t the Apostles warn the elect in the first century of the soon return of Jesus? Who are we to say they were wrong? Were any of us really **sent** (Amos 3:7) to do that? Weren’t the writers of the New Covenant Scriptures **sent** by the Lord (Amos 3:7)? Why do so many today believe **they** are sent to tell us the Apostles, whom the Lord DID send (Amos 3:7; Matthew 28:19-20), were wrong? Why do so many wannabe authorities want to tell us the APOSTLES were wrong?
 See Vincent’s Word Studies and Cambridge Bible for Schools and Colleges.