James Madison is the man who actually wrote the 1st Amendment. If you want to know what he thought about church/state separation, there’s no better thing to read than Memorial and Remonstrance
Memorial and Remonstrance
June 20, 1785
To the Honorable the General Assembly of the Commonwealth of Virginia
A Memorial and Remonstrance
We the subscribers, citizens of the said Commonwealth, having taken into serious consideration, a Bill printed by order of the last Session of General Assembly, entitled “A Bill establishing a provision for Teachers of the Christian Religion,” and conceiving that the same if finally armed with the sanctions of a law, will be a dangerous abuse of power, are bound as faithful members of a free State to remonstrate against it, and to declare the reasons by which we are determined. We remonstrate against the said Bill,
Because we hold it for a fundamental and undeniable truth, “that religion or the duty which we owe to our Creator and the manner of discharging it, can be directed only by reason and conviction, not by force or violence.” The Religion then of every man must be left to the conviction and conscience of every man; and it is the right of every man to exercise it as these may dictate. This right is in its nature an unalienable right. It is unalienable, because the opinions of men, depending only on the evidence contemplated by their own minds cannot follow the dictates of other men: It is unalienable also, because what is here a right towards men, is a duty towards the Creator. It is the duty of every man to render to the Creator such homage and such only as he believes to be acceptable to him. This duty is precedent, both in order of time and in degree of obligation, to the claims of Civil Society. Before any man can be considerd as a member of Civil Society, he must be considered as a subject of the Governour of the Universe: And if a member of Civil Society, do it with a saving of his allegiance to the Universal Sovereign. We maintain therefore that in matters of Religion, no man’s right is abridged by the institution of Civil Society and that Religion is wholly exempt from its cognizance. True it is, that no other rule exists, by which any question which may divide a Society, can be ultimately determined, but the will of the majority; but it is also true that the majority may trespass on the rights of the minority.
Because Religion be exempt from the authority of the Society at large, still less can it be subject to that of the Legislative Body. The latter are but the creatures and vicegerents of the former. Their jurisdiction is both derivative and limited: it is limited with regard to the co-ordinate departments, more necessarily is it limited with regard to the constituents. The preservation of a free Government requires not merely, that the metes and bounds which separate each department of power be invariably maintained; but more especially that neither of them be suffered to overleap the great Barrier which defends the rights of the people. The Rulers who are guilty of such an encroachment, exceed the commission from which they derive their authority, and are Tyrants. The People who submit to it are governed by laws made neither by themselves nor by an authority derived from them, and are slaves.
Because it is proper to take alarm at the first experiment on our liberties. We hold this prudent jealousy to be the first duty of Citizens, and one of the noblest characteristics of the late Revolution. The free men of America did not wait till usurped power had strengthened itself by exercise, and entagled the question in precedents. They saw all the consequences in the principle, and they avoided the consequences by denying the principle. We revere this lesson too much soon to forget it. 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 three pence only of his property for the support of any one establishment, may force him to conform to any other establishment in all cases whatsoever?
read all of it here