Realistically, the AI can research Priesthood in a reasonable time with just the city tile, but building it changes things. If the city tile has even just two hammers, it'll take only 75 turns, which is doable in a game. 3 hammers (on a plains hill), 50 turns. 4 hammers (on a resource), 38 turns. 4 hammers (on a resource that is marble), 19 turns. Makes you think eh? Thinking again, if the case was 4 hammers and marble access, you could call peace to let the AI settle another city and raze the first.