I'm sure it got mentioned, but getting any resources as soon as possible is always preferred over getting the same amount over time or at a later date. So, while it might be true that building a lumber mill will make more total production over the long term (without even considering conditionals like production used to make archers and then those archers capture cities and then those cities easily recoop the production of those archers in both raw production and opportunity costs), the fact of the matter is that 100 production is much more valuable on turn 15 than it is on turn 150. Faith isn't any different. A good couple of chops with Magnus in place can get you missionaries much, much sooner than you would otherwise. Basically, the advantage isn't raw resources; it's time.