The Mission of the Church in Philadelphia is to be a replica of the New Jerusalem
Traditionally, our understanding of the letter to the Church in Philadelphia is limited to noticing a few key facts. We point out that this church was one of the few churches in Revelation that was not criticized by the Lord. We also observe that the word "Philadelphia" means brotherly love, indicating that the people of this church must have loved one another. We often use the idea of the church in Philadelphia as a metaphor for revival.
These are all helpful observations. However, let's not forget that the theme of the book of Revelation is to bring forth a mature Bride of Christ who can judge angels, ushering in God's ultimate judgment on the world. Therefore, we know that God’s work in Philadelphia will somehow contribute to the maturity of the Bride of Christ. The New Jerusalem, the holy city that descends from heaven, is the Bride of Christ. Revelation 21:2 says that the New Jerusalem is like a bride adorned for her husband. The Lord’s promise to the overcomers in Philadelphia is that they will not only become pillars in the temple of God, but they will also have the name of God’s city (the New Jerusalem) written on their foreheads (Revelation 3:12). From these verses, we see that God wants the church at Philadelphia to become a model of the New Jerusalem.