"The key of David," brings to view the prediction of that which was to be laid "upon his shoulder;" so that "he shall open, and none shall shut; and he shall shut, and none shall open," Isa. 22:22. A key symbolizes that which will open or unlock, or will close fast: therefore said the Saviour, "I ... have the keys of hell and of death." By virtue of this power, an open door was set before the church of Philadelphia, which no man should be able to close.
The Jews in Philadelphia, who had claimed to be the only true church of God, but who were in reality of the synagogue of Satan, were to cease their opposition to the Christians, and to seek instruction and protection from them--recognizing the love of God to Gentiles as well as to Jews. History is silent respecting the fulfilment of this; but there is no reason to suppose that it was not literally fulfilled.
The "hour of temptation," which was to "come upon all the world, to try them that dwell on the earth," was to be one of peculiar trial. Some suppose it had reference to the persecution under Trajan, which was more severe and extensive than those under Nero, or Domitian: and others that it was the Mohammedan delusion. In such times there are peculiar temptations to apostatize, and the less faithful are in more danger of apostasy than others. But because the Philadelphian church had been faithful thus far, they were to be kept from that trying hour. When the scourge of Mohammedanism swept over all the other churches of Asia, this church maintained its integrity. Says Gibbon: "Among the Greek colonies and churches of Asia, Philadelphia is still erect, a column in a scene of ruins. At a distance from the sea, forgotten by the emperors, encompassed on all sides by the Turks, her valiant citizens defended their religion and their freedom above fourscore years, and at length capitulated with the proudest of the Ottomans." Philadelphia is still the seat of an archbishop, and contains from six hundred to seven hundred Greek houses, and several places of Christian worship. "The Lord knoweth how to deliver the godly out of temptations," 2 Pet. 2:9.
They are encouraged to constancy by the prospect of the coming coronation day, when "the Lord; the righteous Judge shall give" a "crown of righteousness," "unto all them that love his appearing," 2 Tim. 4:8. He has said "Be thou faithful unto death, and I will give thee a crown of life" (2:10); and therefore "when the chief Shepherd shall appear, ye shall receive a crown of glory that fadeth not away," 1 Pet. 5:4.