Being a lay member of a secondary cult is a useful way to gain training and spirit magic, without binding yourself deeply to the god and its cult.
So for example, IMG, most full-time warriors in Dragon Pass (and Pavis) are lay members of Humakt. That gets Bladesharp, Fireblade, Protection, Parry, Coordination, Vigor, Strength, and all sorts of other good combat-oriented spells. You also get discount training in sword and other combat skills.
All the lay member of Humakt really needs to do is follow the Code of Honor:
Always fight other members fairly.
Honor the fallen.
Maintain strict truth and confidence with one another.
You don’t get any Rune magic as a lay member, but again this is just a secondary cult for your character. But this lets you be an initiate of Yelmalio or Lhankor Mhy or whoever, and still have access to useful combat magic.
