Sardis was probably occupied first by the Hittites, and then by the Lydians. About 550 BC, the most famous Lydian king, Croesus (known as the richest man in the world) was besieged by Cyrus the Persian. Croesus, who had become apathetic and lazy, fortified himself in the acropolis of Sardis, and Cyrus could not capture him.
One afternoon, a Persian soldier named Lagoras watched a Lydian soldier sneak down the back wall of Sardis in order to retrieve his helmet that had fallen. Deducing that a secret trail existed, Lagoras told Cyrus, whose army snuck up that little path at night, surprised the Lydian army, and conquered the city. Croesus hadn't been as careful about the city's defense as he should have been, and he and his people paid for that mistake with their lives.
This story relates closely to the strong metaphor the apostle John used when he warned the church of Sardis: "I know your deeds; you have a reputation of being alive, but you are dead. Wake up!... But if you do not wake up, I will come like a thief, and you will not know at what time I will come to you." (Rev. 3:1-3).
Audio: John's Warning Letter
Given the history of their city, the Christians in Sardis understood this imagery all too well. John, drawing on this common experience of Sardis, was probably implying that they needed to stay watchful and alert in order to maintain their Christian walk. Dr. George Barna, contemporary Christian market research analyst, produces evidence supporting his belief that the biggest problem in modern Christianity is spiritual complacency. Similarly, John was warning the people of Sardis to be careful to stay watchful, alert, and alive to what was going on in their lives, otherwise they might become apathetic or make a mistake that would cause their witness to a watching world to collapse.