These are the remains of three massive pillars that once belonged to the Byzantine Church of St. John the Theologian, located close to the center of the town. The original church had a multi-domed roof supported by six pillars. The inhabitants of Philadelphia suffered much from the many earthquakes that struck their city, one of which destroyed this church also.
Some mistakenly identify these pillars with the promise of Jesus that those who overcome will be made “a pillar in the temple of my God” (Revelation 3.12). The promise will undoubtedly be fulfilled, but these pillars did not exist at the time when the letter to Philadelphia was written and should therefore not be referred to in connection with this promise.