The 'normal' people in the Telanthan Army, once off duty, often huddle in small groups and talk in hushed whispers about mighty men and women, the heroes of Telantha. And these heroes, when they are off duty - they would huddle in small groups and talk in hushed whispers about the people who were selected to become the Temple Guard. They are beyond the best of the best.
Before the Great Dragon left the city (a few years prior to the Darkness) they acted as his personal guard - not that he really needed a group of guards, but they acted as his will in and beyond the temple. They were high enough in the social and military hierarchy that they could speak and their words were assumed to be the words of the Great Dragon.
The mighty golden dragon himself had hand picked each guard several generations ago, and since then, the only way you could be inducted into the temple guard was to be selected by the Great Dragon or you were a descendant of the original guards. However, since the dragon left, it became necessary to 'bend the rules' a little.
Many of the temple guard died in the first wave of the cataclysm, they were the best fighters and mages in the city, so naturally they spearheaded an attack against the demon hoards in the early months when they thought the fight could be won by simply killing the enemy.
Many died, leaving the guard understaffed (though comparatively speaking even with fewer numbers the guard were still more than enough challenge to repel any intention of rebellion against the faith. (They don't respond to political threats unless whatever threatens the rulership of Telantha also threatens the faith, which until the city filled with refugees, was followed by about 95 percent of the population of the city).
Regardless, the Guard are willing to accept certain extraordinary citizens into its ranks. However I do mean extraordinary in every sense of the word, they have to be outstanding combatants or mages, and above all be faithful to the great dragon. To become one you'd need to somehow prove your ability in combat in a practical way- they won't just spar with you to test your mettle, you'd need to do something heroic above and beyond any test.