Desperate faces turned to the Sun
Once the The Fires of Extinction had decimated almost all of the Haendorian population, the few survivors of the scattered Southern settlements found solace in one another. These were difficult times, for their heavy dependence on the wealth of the Great Cities had suddenly been cut off, and they had to start their civilization all over again.
It took the survivors several hundred years to begin civilization again, and as they did they migrated further South; the land of the Sun. Because of this, they naturally began to shift their worship more and more towards the sun. The organization only became official around this time.