Philadelphia was a city 30 miles southeast of Sardis in Asia Minor. It was founded by Attalus II Philadelphus of Pergamos, to be an "open door" for Greek culture. It was destroyed by an earthquake in AD 17 and rebuilt by Emperor Tiberius. Philadelphia specialized in textile and leather production.
The letter to the church of Philadelphia is the 6th one written to the 7 churches in Revelation, chapters 2 and 3, and it receives no reproach. Christ tells the church they have kept his word and have not denied his name, and the assembly of Satan will fall prostrate at their feet. Christ will keep them safe in the time of trial that is going to come to the whole world to test the inhabitants of the earth. The modern city of Alasehir is built over the ancient site.
The other six churches that are mentioned in this part of the book of Revelation are Ephesus, Laodicea, Pergamos, Sardis, Smyrna, and Thyatira.