A New Song
“9 And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;
10 And hast made us unto our God kings and priests: and we shall reign on the earth.”
(Rev 5:9-10 KJV)
The new song is representative of new life that Jesus purchased with His own blood. While the Tribulation period is about to start, John sees those in Heaven worshiping God and recognizes that they came out of every kindred, and tongue, and people, and nation. This diverse group is the Church. We see in verse 10 they are kings and priests and shall reign on the earth. We saw this description given to the Church in Revelation chapter 1.
“5 And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
6 And hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen.”
(Rev 1:5-6 KJV)
It could be that the rapture has taken place and the Church is in Heaven before the Tribulation begins, or they could be the Church members that have gone on to be with Christ and are awaiting in Heaven until the rest of the Church joins them.