Everytime you plunder a city you will receive resources, but you don't always get faith. The amount of faith you get is determined on the power level of the city you attack. The less the power level the less faith you get. If you are farming for faith you should be attacking cities with at least 30k power. You can attack cities with less but the amount of faith you receive really diminishes. It's really inefficient to farm a city that gives you 1 faith every attack when you can simply attack a city that will give you 10,000. When I farm for faith I try to go with cities over 200k power that aren't in alliances.
You only really need as much faith as you have for gods, but it's nice to try to get some for your alliance's Zues statue as well.