Grace, John writes that when the seventh angel begins to sound that there should be time no longer, is also when the mystery of God He has declared to His servants the prophets should be finished. When trying to determine exactly what the mystery is, we need to remember it will be finished when time is no more.
Revelation 10:5-7 (KJV) And the angel which I saw stand upon the sea and upon the earth lifted up his hand to heaven, And sware by him that liveth for ever and ever, who created heaven, and the things that therein are, and the earth, and the things that therein are, and the sea, and the things which are therein, that there should be time no longer: But in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished, as he hath declared to his servants the prophets.
The mystery, according to Paul, is that Gentiles of faith would be included in salvation, and be part of the Kingdom of God with the Jews of faith as the Gospel is proclaimed unto all nations. Prior to the advent of Christ, it was believed that salvation and the Kingdom of God was exclusively for the Jews of faith. Most Gentiles, prior to the sending of the Gospel to them, had no understanding of Messiah who would come and through faith in Him provide a way of eternal life in the Kingdom of God for even them if they believe in Christ.
Romans 11:25 (KJV) For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
Romans 16:25-26 (KJV) Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith:
Ephesians 3:3-7 (KJV) How that by revelation he made known unto me the mystery; (as I wrote afore in few words, Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power.
Ephesians 3:8-9 (KJV) Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ; And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:
Colossians 1:26-27 (KJV) Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints: To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
1 Timothy 3:16 (KJV) And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
The same one Gospel of Christ, once hidden from other nations, made known through the Law and the Prophets of old to Israel alone. Now, since the advent of Christ has been revealed to peoples throughout the earth, and whosoever ever by grace through faith believes the Gospel of Christ shall have eternal life through Him.
When the spiritual Kingdom of God in heaven is complete, after the last Gentile to be saved comes into the Kingdom, then time given this earth for proclaming the Gospel shall be no more.