According to Rev. 4, John is taken to heaven, and he sees 24 elders already seated in the throne room of God dressed in white raiment with gold crowns on their heads.  While some theologians believe that 12 of the elders represent the 12 tribes of Israel and the other 12 elders represent the 12 disciples, is it not possible that all 24 represent born again believers who are overcomers evidenced by their white raiment and golden crowns who are priests on a rotating basis serving in the throne room in heaven like the 24 priests who served in the temple in Jerusalem on a rotating basis once a year.  Since we will have immortal bodies, we will all be able to serve in the throne room on a rotating basis during eternal life which never ends.

Rev. 1:5-6 says, "...Unto him that loved us and washed us from our sins in his own blood And hath made us kings and priests unto God and his father; to him be glory and dominion for ever and ever. Amen." Rev. 5:21 says, "To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne." 

Also in regard to the 24 elders, Rev. 5:9-10 says, "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; And hast made us unto our God kings and priests: and we shall reign on the earth."

Matthew 24:14 says, "And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come."  Mark 13:10 says, "And the gospel must first be published among all nations."

Of course, there are more than 24 nations in the world, but the gospel must have been preached to all nations in the world in order for the end to come and along with it the resurrection-rapture at which time the redeemed are taken to heaven to God's throne room.

If the 24 elders represented the 12 tribes of Israel and the 12 disciples, they would have all come from the nation of Israel.  And if the 24 elders just came from the 7 churches in Asia Minor, the elders would have been from the nation of Turkey.  Therefore, the 24 elders who came from all the nations of the world may represent overcomers from the beginning of the church down through the ages until the present time in order for the gospel to be preached to all the nations before the end came.