Sardis was a town in the west of Asia Minor (Turkey), 50 miles east of Smyrna, and 30 miles southeast of Thyatira. It was formerly a capitol of Lydia, and became part of the Roman Empire in 129 BC. Paganism flourished in Sardis. The Christian community was small and weak, and a lot of them returned to their original religions.
The letter to the church of Sardis was the 5th written to the 7 churches in Revelation, chapters 2 and 3. It proclaimed that the church of Sardis was dead, and told the Christians there to be watchful and to strengthen what remained of their community. The letter also said that there would be a few people in Sardis who will walk with Christ.
The other six churches that are mentioned in this part of the book of Revelation are Ephesus, Laodicea, Pergamos, Philadelphia, Smyrna, and Thyatira.