> I'm just wondering what the effect of a dopamine reducing drug would
> be on your average believer - would they lose their faith completely?

You can find that out by studying long term users of the following
drugs: opium and opiates, ephedrine and similar, and nicotine. All
three groups act on the "reward centre" of the brain by preventing the
re-uptake of dopamine. This means that, at normal levels of
production, the brain gets a major dopamine hit as the levels of the
neurotransmitter rise. Since dopamine is the primary chemical
associated with the "reward centre" of the brain, people feel "high"
as their pleasure reflex is tripped continuously.

Unfortunately, having adaptive brains, we produce less dopamine to
compensate, so more and more drug is necessary. As a simple
experiment, find a hopeless drug addict - say a 60-a-day smoker, and
withdraw their favourite drug for 24 hours. Then ask them if they
believe in a god :-)