I read a commentary that ancient Philadelphia was a place that was subject to earthquakes. A lot of people got hurt if they happened to be in a columned temple at the time of an earthquake.
The meaning could be; That the overcomer(s) would be a pillar in G-d's Holy Temple with G-d's Name written upon them....and....that they would never have to "go out" or run out of temple again because the pillars are secured in place with G-d's assurance of support.
This is, at least, a very probable explanation!!
Regards to Mike, Paul and many thanks to John Tng.