To the Church in Philadelphia
7 “To the angel of the church in Philadelphia write:
These are the words of him who is holy and true, who holds the key of David. What he opens no one can shut, and what he shuts no one can open. 8 I know your deeds. See, I have placed before you an open door that no one can shut. I know that you have little strength, yet you have kept my word and have not denied my name. 9 I will make those who are of the synagogue of Satan, who claim to be Jews though they are not, but are liars—I will make them come and fall down at your feet and acknowledge that I have loved you. 10 Since you have kept my command to endure patiently, I will also keep you from the hour of trial that is going to come on the whole world to test the inhabitants of the earth.
11 I am coming soon. Hold on to what you have, so that no one will take your crown. 12 The one who is victorious I will make a pillar in the temple of my God. Never again will they leave it. I will write on them the name of my God and the name of the city of my God, the new Jerusalem, which is coming down out of heaven from my God; and I will also write on them my new name. 13 Whoever has ears, let them hear what the Spirit says to the churches.
Whereas the message to the church at Smyrna was absent of any commendation, the church at Philadelphia is almost given nothing but praise. The metaphor for the church is that God has created an "open door" that no one can shut. On the flip side, no one can open the door that He has shut. Oftentimes the church has used the term "open door" as an opportunity to go to the mission field, or undertake a special program of evangelism. The idea is that if God has opened a door if we have the faith to walk through it, we will be blessed. And Paul definitely uses it in this sense of the word.
But in the context, the "open door" seems to be an open door to God's salvation and entrance into heaven. The open door will be kept open by God for those who hold on to their faith. With reference to David it is the Messianic kingdom promised to those who believed in the Son of David, and the door for reigning with the King of kings forever.
Jesus sees that they have little strength in a spiritual sense, as they are battling against persecution from who he calls "a synagogue of Satan". These were Jews who denied Jesus and subsequently closed the door on His being their Messiah. There were also Gentiles, who became Jews to avoid the Roman persecution. So this could pertain to either of these groups or both.
The message concludes with the promise to those who hold on of a crown in heaven that will last forever. They will also escape the hour of trial coming on the whole earth. Some regard this as the rapture of believers, who will escape the final tribulations on the earth before Christ returns in great and promised glory. So the open door is a door that leads to all of which we will inherit as God's chosen believers.
One way to think about this is that there is still an open door for all people to come to salvation. Jesus says I am the door to salvation. Until the final day, the door remains open for all who would enter through it by faith. Often we might close the door to someone before God would. In this sense the church is an open door for people to come and find salvation through the hearing of the Word. And our very lives can be open doors that Jesus can use to invite people to come to know him by our witness and testimony. The bottom line is we should never shut a door that God may want to keep open. We can keep working as an open door to God's kingdom until the day God chooses to shut it.