Because the Bill implies either that the Civil Magistrate is a competent Judge of Religious Truth; or that he may employ Religion as an engine of Civil policy. The first is an arrogant pretension falsified by the contradictory opinions of Rulers in all ages, and throughout the world: the second an unhallowed perversion of the means of salvation.

James Madison (1751-1836) American statesman, political theorist, US President (1809-17)

“A Memorial and Remonstrance Against Religious Assessments,” letter to the Virginia Assembly (20 Jun 1785)

On a proposed law to have the state financially support "Teachers of the Christian Religion."

Full text.