Philadelphia means ‘the city of brotherly love’. Whether that was true of the city is unlikely. Yet it was true of the church there because Jesus finds no fault within it. It is the second of the seven churches to have this level of commendation, along with the church in Smyrna. This means that the city of brotherly love had a community of brotherly love within it.
In describing himself, Jesus says that he is both God and man, which is how we should think of him when focusing on what he has done since his incarnation. His deity is seen in his description of himself as the one who is holy and true. Yet they would also be accurate descriptions of his human nature. As the one who is holy and true, he cannot tell a lie, which would be of great comfort to the small church in Philadelphia as he makes promises to them about their future.
The significance of him having the key of David is connected to what is described in Isaiah 22:20ff where the prophet reveals that the Lord is going to remove Shebna from his place of power because of his unfaithfulness and Eliakim in that position of power. The key symbolised the power that Eliakim would have. They revealed that the king had given him authority to act. Jesus means by this description that as the Messiah he has been given full authority by God the Father.
The person who had the key in the palace in Jerusalem could go into any room he wished. He also allowed or prevented other people from having access to those places. In other words, nothing could happen without his decision or permission. In a far higher sense, nothing happens without the permission of Jesus. He has access to everything in heaven, including the details of God’s eternal plan. Therefore, he can make definite promises to the church in Philadelphia.
It is obvious that Jesus wanted to communicate this information to his people. He wanted them to get the benefits that come from knowing that this is his role. It would mean that whatever happened to them should be connected by them to the key that Jesus possesses as God’s Messiah. The same is true for us.