Who said the following:
"because experience witnesses that ecclesiastical establishments, instead of maintaining the purity and efficacy of religion, have had a contrary operation. during almost fifteen centuries has the legal establishment of christianity been on trial. what have been its fruits? more or less in all places, pride and indolence in the clergy; ignorance and superstition in the laity; in both, superstition, bigotry, and persecution."
Hint: it *wasn't* karl marx or charles darwin.
To piggyback on arachnid's correct answer, James Madison was arguing against a bill in the Virginia assembly, which endorsed state support for all Protestant factions by using public assessment to provide for religious teachers. Madison spelled out several arguments against this bill, one of which is quoted here. This document makes it very clear where Madison stood on the issue of church involvement in state affairs, and is one of many defining arguments put forth by American founding fathers in favor of separation of church and state.
James Madison, 4th president of the united states, when arguing for separation of church and state: http://press-pubs.uchicago.edu/founders/documents/amendI_religions43.html
I'd heard that quote before, but hadn't seen the context from which it was taken. It's an excellent argument - thanks for pointing me to it!