The Six Guardians of the Holy Grail belonged to a cult which worshiped the Goddess. When Galahad and then Arthur came seeking the Holy Grail, they tested them in heart, mind, spirit and body. One should pass each one of them before the Goddess relinquish the guardianship of her sacred Cup of Regeneration to him.
It is not known if the cult was composed only of those six, or whether they belonged to a greater cult.
It is not clear what they guarded: either their old faith, or the Holy Grail.