James Madison | The First Amendment Encyclopedia

The target of the pamphlet was a bill before the Virginia Assembly supported by Gov. Patrick Henry that would have laid a general tax to pay Christian teachers a modest salary. The bill would not have established any one denomination (all Christian churches were eligible for the funds), but it would have made Christianity the established religion of the state. 

Madison considered the bill a "dangerous abuse of power"; he reasoned that if government could establish Christianity over other religions, then it would also have the power to elevate one Christian group over another. Madison believed that religion was a matter of individual conscience and that giving legislators control over religious belief would inevitably lead to violation of other basic rights: "It is proper to take alarm at the first experiment on our liberties." Madison succeeded in defeating the religious assessment bill and then spearheaded passage, in 1786, of Thomas Jefferson's Virginia Statute for Religious Freedom...

https://www.mtsu.edu/first-amendment/article/1220/james-madison