The Letter to the Church at Philadelphia. Philadelphia was about 30 miles SE. of Sardis. Like Sardis it was subject to frequent earthquakes, and consequently never attained any great size. The condition of the church seems to have been satisfactory; the difficulties arose from Jewish rather than pagan opponents.
Revelation 3:7. that hath the key of David: cf. Isaiah 22:22, where the key of the house of David is given to Eliakim after the deposition of Shebna. The key was the symbol of office, especially of the Treasurer's or Chancellor's office. It is Christ who here carries the key which indicates His authority over the House of God.
Revelation 3:8. door opened: probably the door of opportunity (cf. 1 Corinthians 16:9; 2 Corinthians 2:12). Possibly there is an allusion to the geographical position of Philadelphia, which was situated in the centre of Asia, and so had abundant opportunities of evangelizing the neighbouring districts.
Revelation 3:9. synagogue of Satan: as at Smyrna (cf. Revelation 2:9), the opposition comes from the Jews.
Revelation 3:10. the word of my patience: the teaching which emphasized and held up as an example the endurance of Christ. the hour of trial: the hour of testing, i.e. the persecution.
Revelation 3:12. a pillar in the temple: the man who remains steadfast in the hour of testing will become a pillar (cf. Galatians 2:9, of James and Cephas and John), i.e. one of the chief supports of the Church of God. name of my God: three names are written on the pillar man, (a) the name of God (cf. Numbers 6:27), to indicate that his life was consecrated to the service of God; (b) the name of the New Jerusalem, to indicate that he was a citizen thereof; (c) the new name of Christ (cf. Revelation 19:12), to indicate that he was to share in His victory. For the description of the New Jerusalem cf. Revelation 21:2. See also Galatians 4:26 *.