A Great Soul is a follower that has served the faith/Principal honestly and through significant work on its behalf. Essentially, an icon representing their faith, whatever it is. The vast majority of their Essence is tied into their Divine Principal, their Conviction, and the utter devotion to the Abiding Litany of the faith. Their belief is their shield and sword, they eschew material accouterments to augment their power – such enhancements are accumulated and bound into themselves in their service to their faith.
While not all of them are exactly the same for all faiths, there is a rough template.