Palms of Tn'kaneid:

The owner of this wonder gains significantly increased chances of success when attempting to religiously convert a foreign nation that already shares their primary religion. All pops created by the owner of this wonder are always of the primary religion, and all pops gained via other methods have a 50% chance of adopting the religion. Each session, there is a 25% chance that a random nearby pop converts to the owner's primary religion.