In Age of Empires 1 you need the "Monotheism" research.
In Age of Empires 2, you need the "Redemption" tech.
In Age of Empires 3, Priests are just medics, and can't convert.
Sounds like you know your AoE. What mission was it where you start with only 2 monks near a brown(?) village. you had to convert villagers to start building your own settlement and stuff.
There was a mission in AoE, the first, the babylon campaign. You started with one monk with a golden brown gguy on the right and i beefy red guy on the left. You had one monk. There was no mission in AoE with 2 monks but a couple with 1 monk and a convertible enemy monk.
There was mission with 2 monks start, but it was in first AoE demo. Demo got it's own missions, 5 or 6, that were not in a full game, similar to demos of Starcraft and Warcraft 3.