favored souls

Divine spell-casters are all either theurgists or favored souls. Favored souls are born with divine magic powers and theurgists  gain their magic later in life.
Most favored souls are marked by their deity at the moment of conception or while in the womb. A small number of favored souls are infused with divine power as children. A disproportionately high number of favored souls are the children of couplings between mortals and spirits or they are the legacies of such half-spirit/half mortals.
Favored souls may be marked at a very young age, but they do not normally gain access to their magic powers until adolescence. Some young favored souls are left to fend for themselves but most are visited by a spirit guide in their early life who explains to them who they are and what they can do. A lucky few favored souls retain their spirit guides their whole lives.
A lot of favored souls are conceived during their deity's holiday celebrations. Favored souls are usually far more likely to be born to the most devout worshipers of their deity than to those who are indifferent or disdainful of the deity though exceptions, occur, especially during the deity's Zodiac year or month.
Favored Souls are slightly more likely to be born in their deity's Zodiac month than outside it. Favored souls are generally much more likely to be born inside their deity's Zodiac year than outside it.
Some parents will go through great efforts to have children born in certain Zodiac years or they will take efforts to avoid having their children born during certain Zodiac years.

Rebellious Favored Souls

Unlike theurgists, favored souls may never become excommunicated. Most favored souls are paragons of their deity's values. Many Scarterrans, many favored souls even, believe that favored souls do not truly have free will and that a favored soul can never stray from their deity's path. Even if it doesn't look like a favored soul is following their deity's wishes, their actions will advance their patron's goals in the long run, at least that's what many storytellers and sages say.
This is actually false. A favored soul can turn away from any or all of their divine patron's teachings for any or no reason at all and the Nine are unable to do anything about this other than to send a spirits to talk to the erring favored soul or try to kill them, but the Nine  cannot remove the powers from a favored soul that displeases them and very few of them want to go as far as to destroy their wayward children.
This is not widely known because very few favored souls want to rebel against their patron.

Favored Souls by Deity

Maylar, Zarthus, and Greymoria tend to have lots of favored souls.
Hallisan and Khemra have very few favored souls.  Phidas has zero favored souls. 
Nami, Mera, and Korus tend to fall in between the two extremes with modest number of favored souls compared to the others.