Having finished His letter to Sardis, Jesus says in Revelation 3:7-8,
7 “And to the angel of the church in Philadelphia write: ‘The words of the holy one, the true one, who has the key of David, who opens and no one will shut, who shuts and no one opens.
8 “‘I know your works. Behold, I have set before you an open door, which no one is able to shut. I know that you have but little power, and yet you have kept my word and have not denied my name.
In this letter to the church in the city of Philadelphia (now Alaşehir in western Turkey), Jesus begins by introducing it like this: "The words of the holy one." Jesus is the One who is set apart far above all rule and authority and power and dominion (Ephesians 1:21).
He's also "the true one." As opposed to the idols in Philadelphia, Jesus is the true and living God. Furthermore, He has "the key of David." Isaiah 22:22 prophesies, "And I will place on his shoulder the key of the house of David. He shall open, and none shall shut; and he shall shut, and none shall open." Christ has been given the keys to the kingdom.
He continues to say of Himself, "...who opens and no one will shut, who shuts and no one opens" (v. 7). If Jesus opens a door, it remains open. If Jesus shuts a door, it remains shut.
What has likely happened in the church in Philadelphia is that the church has somehow been excluded. Perhaps they were excommunicated from the synagogue, for example. Jesus assures them that even if they have been shut out by unbelievers, Jesus Himself has the keys to the door that really matters.
He goes on in verse 8, "I know your works." He knows that they've been faithful to Him. "Behold, I have set before you an open door, which no one is able to shut." Based on verse 12, this is probably talking about entrance into heaven, the New Jerusalem. Christ opened the door to His faithful people, and no one would be able to shut them out, even if they shut out the Christians in a temporal, earthly way.
The church in Philadelphia was small and weak: "I know that you have but little power" (v. 8). Regardless, Jesus says of them, "...and yet you have kept my word and have not denied my name." What an awesome thing to be remembered by--being a small and weak church, yet remaining faithful to Christ in spite of the world's rejection.
This should be our aim as a local church. If we get shut out by the world, we need to remember that Christ has the key to the door that really matters, and for we who believe in Him (and thus serve Him faithfully), He has opened that door to us, and no one will be able to shut it. So, let them shut us out. And even if we have little power, may we keep His word and not deny His name. The door that Jesus provides is open. No other door matters.