Mission is the sixth mark of the church (Rev. 3:7–13).
Before Jesus Christ ascended into heaven, he gave his disciples what we now call the Great Commission: “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age” (Matt. 28:18–20). This commission consists of three elements: the fact of Christ’s authority, the command to make disciples, and the promise of Christ’s presence.
We see the same three elements at work in the letter to the church in Philadelphia.
Fact: “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.” Jesus Christ is God’s key master, who opens doors of opportunity for his mission-minded followers.
Command: “Behold, I have set before you an open door which no one is able to shut.” Although Jesus Christ has the power to shut doors of opportunity in such a way that no one can open them, he does not use that power in his churches. Rather, he only opens doors so that we might “go” and “make disciples.”
The church in Philadelphia was providentially prepared to walk through such an open door. John Stott comments: “Philadelphia was situated in a broad and fertile valley which commanded the trade routes in all directions. Sir William Ramsay wrote that the intention of the city’s founder had been to make it a centre for the spread of Greek language and civilization. ‘It was a missionary city from the beginning.’ So it may be that Christ was intending that what Philadelphia had been for Greek culture, it was now to be for the spread of the gospel.”
Promise: “I will keep you from the hour of trial that is coming on the whole world, to try those who dwell on the earth.” A missionary church never seeks out conflict with others, but conflict comes to it nevertheless. Wherever the church shares the good news of God’s love, powerful interests oppose it. At the church in Pergamum, that powerful interest was the Roman imperial cult and the ius gladii (“power of the sword”) that enforced it. At the churches in Smyrna and Philadelphia, that powerful interest was the Jewish synagogue, which Jesus refers to as “the synagogue of Satan.”
I read those four words with trepidation. Looking backward from Auschwitz at the relationship of Jews and Christians, I see how Gentile Christians used such descriptions to hatefully, wrongfully, and unjustly persecute Jews down the centuries. Such persecution was, is, and always will be a sin. But to understand these words in their historical setting we must remember that Jesus Christ, the letter writer, is a Jew, as is John, his amanuensis. Also, in the first century when Revelation was written, Judaism was a large community of faith but Christianity a small one. Auschwitz is an awful reminder that for centuries Christians persecuted Jews. Philadelphia is a small reminder that for a brief time, persecution flowed in the opposite direction.
But if we understand the mission of the church rightly, we will see that persuasion, not persecution, is the way the church of Jesus Christ should accomplish its mission. Christ has set before us an open door to tell others of his love for them. Sometimes, such evangelism will result in conflict. Knowing that Jesus Christ is with us, let us go through the door anyway.
 John Stott, The Incomparable Christ (Downers Grove, IL: InterVarsity Press, 2001), 180.