Christ’s letter to the church in Philadelphia can be found in Rev 3.7-13. My study of that letter indicates that as in much of the early Christian world, the Christians at Philadephia were worshipping in a Jewish synagogue. The context of Rev 3.7-13 clearly indicates that the Christians are suffering severe hardship under the leaders of the synagogue and have most likely been put out of the synagogue. By the time Ignatius writes his letter to Philadelphia, they seem to still be struggling with the same issues (see ch. VI). The letter is historically of great interest (not least because Ignatius clearly defines the heresy of the mysterious Nicolatains) but more importantly it is spiritually edifying. I have excerpted a quote below that did my soul well. In it, Ignatius talks about how the Gospel transcends all the other revelations of God, whether they be in the O.T. or New. God’s supreme and transcendent grace, is for Ignatius, contained in the appearing of Jesus, his death and resurrection. I hope you enjoy it.
The priests indeed and the ministers of the word are good; but the High Priest is better, to whom the holy of holies has been committed, and who alone has been entrusted with the secrets of God. The ministering powers of God are good. The Comforter is holy, and the Word is holy, the Son of the Father, by whom He made all things, and exercises a providence over them all, This is the Way which leads to the Father, the Rock, the Defence, the Key, the Shepherd, the Sacrifice, the Door o fknowledge, through which have entered Abraham, Isaac, and Jacob, Moses and all the company of the prophets, and these pillars of the world, the apostles, and the spouse of Christ, on whose account He poured out his own blood, as her marriage portion, that He might redeem her. All these things tend towards the unity of the one and true God. But the Gospel possesses soemthign transcendent [above the former dispensation], viz., the appearing of our Saviour Jesus Christ, His passion, and the ressurction itself. For those things which the prophets announced, saying, “Until He come for whom it is reserved, and He shall be the expectation of the Gentiles,” have been fulfilled in teh Gospel, [our Lord saying], “Go ye and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.”
-Ignatius, “Letter to the Philadelphians” ch. IX excerpted from The Apostolic Fathers: with Justin Martyr American Edition, vol I pg 84-85