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the metropolis of Lydia in Asia Minor
It stood on the river Pactolus, at the foot of mount Tmolus. Here was one of the seven Asiatic churches (Rev. 3:1-6). It is now a ruin called Sert-Kalessi.
See the Christian archaeological video which describes this city and the cultural context surrounding the early Church: On the Early Church (“The Salt of the Earth,” part of the Faith Lessons video series). “The apostle John used the example of Sardis, a city that overlooked its weak points and thereby enabled invading armies to conquer it, to warn Christians to be alert to their weakness so that they would not compromise their faith.”