Having just finished the Hoarder achievement (after a few failures because some of the details of cult acquisition aren't entirely clear), I thought it might be useful to write up a short guide on the mechanics.
First of all, to be clear, I'm discussing cults unlocked by nations with Fetishist religions (i.e. Sub-Saharan African nations that aren't Muslim). If you are one of these nations, you'll start with 3 different cult choices that are geographically determined.
- Central Africa (Kongo river basin, Great Lakes region, Mutapa region) has 3 cults: Cwezi, Mlira, Mwari
- Madagascar has 3 cults: Enkai, Waaq, Zanahary
- West Africa (Benin, Mossi, Fulo area, etc.) has 3 cults: Roog, Nyame, Yemoja
Edit: With the map rework of the Ethiopia region, the two Fetishist nations there should have 3-6 starting cults (Cwezi, Mlira, Mwari, Roog, Nyame, Yemoja). I have run 2 test games there and had different results (once only getting 3 cults and having the other 3 locked out, once getting all 6 cults). I'll test further and update...
All nations within those specific regions should have the same set of cults.
If you ally or go to war with a fellow Fetishist nation in one of these other areas, you have a chance (MTTH event) of gaining a new cult. Sometimes you can get more than one in a single war. I got all three from the Madagascar region, for example, at the same time upon invading one of the nations there.
The quickest way to get a cult from another nation seems to be by engaging them in battle (either naval or land). You'll likely trigger all 3 cults at once by doing so.
Consequently, a valid strategy as a Fetishist nation is to ally or attack Benin, Kongo or Mutapa, and someone in Madagascar (not the Sunni state). As Benin, for example, you can colonize in Luanda (by Kongo), ally Kongo, colonize in Cape, colonize in Ile Bourbon, and attack Madagascar. Doing so will net you all 9 cults unless something goes wrong (like the Sunni nation unifying and converting Madagascar).
In addition to the traditional African cults, you can gain other cults from heretic and heathen nations. Again, these are gained from allying a nearby (doesn't need to be exactly neighboring but that seems to help) nation or by attacking them. Update: It's now possible to unlock a cult by owning provinces of that religion.
- Buddhist branches (Mahayana, Theravada, Vajrayana): Buddhadharma
- Central American faiths (Nahuatl and Mayan; note: not Inti from South America): Teotl
- Christian denominations (Catholic, Coptic, Orthodox, Protestant, Reformed): Christianity
- Dharmic religions (Hindu, presumably Sikh as well): Sanatana Dharma
- Muslim denominations (Ibadi, Shia, Sunni): Islam
Finally, there are 3 difficult to obtain cults (that you don't need for the achievement). You get these by allying or attacking a nation that owns them (if they are of the official religion of that province). The best way to get it then, is by conquering the territory and returning the core, then allying or attacking the new nation (Semien or Persia) that should follow the faith of their sole province. Edit: It's now possible to unlock a cult by owning the relevant province (Semien, Yazd) before it's converted to a different religion. Update: There are now 2 Jewish provinces in Ethiopia.
- Jewish (only from Semien before it's converted): Haymanot
- Norse (only in Random New World with fantasy options): Freyja
- Zoroastrian (only from Yazd before it's converted): Mazdayasna
If you are going for the achievement, one of your first ideas should be Exploration. You only need 13 cults, so all 9 of the traditional African cults plus 4 of the 5 major world religion cults (and none of the 3 difficult ones) will trigger the achievement for you. Islam is the easiest of the major religions since it's native to Sub-Saharan Africa. Christianity is also easy since the colonists will come to Africa. The Hindu cult could be picked up in Indonesia by starting a colony adjacent to Majapahit and allying them. If Pagarruyung still exists, you can do the same to them for the Buddhist cult, as they are Theravadan. Once you have the cult(s), you could break the alliance(s). The Teotl (Nahuatl/Mayan) one easily comes from colonizing Tamaulipas (in northern Mexico, just north of Totonac). You could ally a Nahuatl or Mayan nation or just declare war on one, then peace them out once you have your cult.
Update: With the Ethiopia region rework, playing as Welayta can be one of the easiest ways to get the Haymanot cult (by invading Ethiopia for one of the two Jewish provinces).
As noted above, battles seem to be the fastest/most reliable trigger for the spread of cults.
Credit to this post that provided some useful info on the breakdown of the traditional African cults.
