This article is from the Atheism FAQ, by mathew meta@pobox.com with numerous contributions by others.
The principle of the separation of church and state is that the state
shall not legislate concerning matters of religious belief. In
particular, it means not only that the state cannot promote one
religion at the expense of another, but also that it cannot promote
any belief which is religious in nature.
Religions can still have a say in discussion of purely secular
matters. For example, religious believers have historically been
responsible for encouraging many political reforms. Even today, many
organizations campaigning for an increase in spending on foreign aid
are founded as religious campaigns. So long as they campaign
concerning secular matters, and so long as they do not discriminate on
religious grounds, most atheists are quite happy to see them have
their say.
 
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