Because the establishment proposed by the Bill is not requisite for the support of the Christian Religion. To say that it is, is a contradiction to the Christian Religion itself, for every page of it disavows a dependence on the powers of this world: it is a contradiction to fact; for it is known that this Religion both existed and flourished, not only without the support of human laws, but in spite of every opposition from them, and not only during the period of miraculous aid, but long after it had been left to its own evidence and the ordinary care of Providence. Nay, it is a contradiction in terms; for a Religion not invented by human policy, must have pre-existed and been supported, before it was established by human policy.

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.