The Missionary City by W. M. Ramsay (Offsite Link)

“Philadelphia was the only Pergamenian foundation among the Seven Cities. It derived its name from Attalus II, 159-138 BC, whose truth and loyalty to his brother Eumenes won him the epithet Philadelphus. The district where it was situated, the valley of the Cogamis, a tributary of the Hermus, came into the possession of the Pergamenian King Eumenes at the treaty of 189. From that time onward the district was in the heart of the Pergamenian realm; and therefore the new city could not have been founded as a military colony to guard a frontier, like Thyatira. Military strength was, of course, never entirely neglected in those foundations of the Greek kings; and especially a city founded, like Philadelphia, on an important road, was charged with the duty of guarding the road. But military strength and defence against invasion were required chiefly near the eastern frontier, far away on the other side of Phrygia, where an enemy should be prevented from entering the realm. Philadelphia was founded more for consolidating and regulating and educating the central regions subject to the Pergamenian kings. The intention of its founder was to make it a centre of the Graeco-Asiatic civilisation and a means of spreading the Greek language and manners in the eastern parts of Lydia and in Phrygia. It was a missionary city from the beginning, founded to promote a certain unity of spirit, customs, and loyalty within the realm, the apostle of Hellenism in an Oriental land. It was a successful teacher. Before AD 19 the Lydian tongue had ceased to be spoken in Lydia, and Greek was the only language of the country” (Ramsay, The Letters to the Seven Churches of Asia, Chapter 27).

On our Holy Days 2000 Page, by following the Pentecost 2000 links,  you may access both audio and transcripts relating the significance of Philadelphia to church history. This series by Fred R. Coulter is entitled The Seven Church Harvest.