Philadelphia was founded in the 2nd century BC by the Pergamese king Attalos II Philadelphos. The name (Philadelphia ="brotherly love") goes back to the close relationship between Attalos and his brother Eumenes II. Philadelphia or Philadelpheia was on the trade route between Sardes, Tripoli, Hierapolis, Laodikeia and Kolossai on the site of today's Alaşehir. "("City of God" or "City of Allah")

The Byzantine walls that once surrounded the city have disappeared except for a few crumbling remains on the northern outskirts. These wall fragments are, like three pillars of the St. John's Basilica built in the 6th century and some remaining remains of an ancient theatre, the only witnesses of this once rich and well-known city.

In the 1st century A.D. an early Christian community gathered here, which is mentioned in the Revelation of John. It was the last Byzantine city in Asia Minor to defend itself against the Ottomans until 1390. When Philadelphia fell, the rest of Asia Minor had already been under Ottoman rule for about 50 years (except for the Empire of Trapezunt, which the Ottomans occupied only in 1461). Under Ottoman rule, the city was renamed Alaşehir

When the Mongols destroyed the Christian communities of Asia Minor, Philadelphia was miraculously saved after Ludwig Albrecht. The inhabitants of the small town are the only ones among the Muslim population of the region to have preserved the Christian faith. A Christian community is documented until at least the beginning of the 20th century.

Like Ephesus, the early Christian church there was one of the seven churches in Asia Minor addressed in Revelation. The Epistle of John 3:7-13 was addressed to the congregation of Philadelphia. In it their persistence during the persecution of Christians was emphasized.

To this day there is a district in Athens called New Philadelphia, which was founded after the population exchange between Greece and Turkey by former inhabitants of Philadelphia in northern Athens.