For those whose faith is true, Jesus promises to use his key authority to their benefit. He gives a two-fold metric to understand his judgment in the matter: 1) the faithful guard (in the sense that they observe or keep) his word, and 2) the faithful do not deny (disavow or repudiate) his name. In the case of the Philadelphians, both are done in spite of the lack of great ability (dunamis). For the non-charismatic that would probably be taken another way, but for those of us who are charismatic, we could see this as referring to a relative lack of miraculous, spiritual power. Perhaps that is encouraging news to those living in an age where our affirmation for fabulous signs and wonders far outstrips their actual occurrence.
The open door cannot refer to opportunities for gospel work (as so many commentators aver), for that would be a far too pedestrian use of Jesus' keys--particularly since the benefactors experience a lack of power and opposition in this world. The open door, therefore, must refer to something in regard to which Christ has unique authority (since none can close what he opens by it). Since these letters have been filled with so many warnings about the things of Christ being closed off to some, context would demand (it seems to me), to see this in regard to all that Christ has been promising by his authority in the midst of these letters. What others written to are shut out from, the Philadelphians are allowed in to.
It is implied in Christ's message to their church, that the faithful Philadelphians were taking some kind of flak from the so-called Synogogue of Satan. That, along with the reference to the Key of David, puts a markedly Jewish spin on this message. That does not make much sense to me historically or in terms of historic epochs, but it must be significant nonetheless. It is ironic that those of that persuasion (call them Judaizers, if you wish) were the ones in fact, who will find themselves ostracized by the one who has the Davidic key.