I only know what I read in biographies. Some were, some weren't, and none of them advocated making this a christian nation.
Who does not see that the same authority which can establish Christianity in exclusion of all other religions may establish, with the same ease, any particular sect of Christians in exclusion of all other sects? That the same authority which can force a citizen to contribute threepence only of his property for the support of any one establishment may force him to conform to any other establishment in all cases whatsoever?
James Madison, A Memorial and Remonstrance Against Religious Assessments, June 20, 1785.