Three Sisters

The members of the Body of Christ compose the Royal Priesthood. Each one has a measure of Christ in him or her. The term "Christ" refers to the "Anointed One," that is, the One anointed with the Spirit of God to serve God as a priest. As such, every human being who has a portion of Christ living in him or her is no longer of the world, just as the Lord Jesus no longer is of the world.
The holy city, the new Jerusalem, is the home of the Royal Priesthood.
There is a great wall separating the Royal Priesthood from the nations of saved people of the earth. The saved people, the "sheep," mentioned in the twenty-fifth chapter of the Book of Matthew, have been brought forward to life on the new earth. They can enter the new Jerusalem and receive eternal life and healing, but they are not of the priesthood. The priesthood is by Divine calling, and no one can take this honor to himself
I do not believe there is any limit on spiritual growth that may be achieved by a member of the nations of saved people. They may enter through the gates into the new Jerusalem, but they must keep the laws of God.
Blessed are those who wash their robes, that they may have the right to the tree of life and may go through the gates into the city. Outside are the dogs, those who practice magic arts, the sexually immoral, the murderers, the idolaters and everyone who loves and practices falsehood. (Revelation 22:14.15)
The people who are not of the priesthood may wash their robes in the blood of the Lamb by repenting of any sin in their life, and then they may have access to the Tree of Life on the banks of the River of Life.
The nations will walk by its light, and the kings of the earth will bring their splendor into it. (Revelation 21:24)
But the members of the Royal Priesthood have been chosen from the beginning of the world. That is why the Apostle Paul does not refer to the believers as Christians, but as "saints" (holy ones).