James Madison on separation of church/state:
The civil Government, though bereft of everything like an associated hierarchy, possesses the requisite stability, and performs its functions with complete success, whilst the number, the industry, and the morality of the priesthood, and the devotion of the people, have been manifestly increased by the total separation of the church from the State (Letter to Robert Walsh, Mar. 2, 1819).
Strongly guarded as is the separation between religion and & Gov't in the Constitution of the United States the danger of encroachment by Ecclesiastical Bodies, may be illustrated by precedents already furnished in their short history (Detached Memoranda, circa 1820).
Every new and successful example, therefore, of a perfect separation between the ecclesiastical and civil matters, is of importance; and I have no doubt that every new example will succeed, as every past one has done, in showing that religion and Government will both exist in greater purity the less they are mixed together (Letter to Edward Livingston, July 10, 1822).
Congress should not establish a religion and enforce the legal observation of it by law, nor compel men to worship God in any manner contary to their conscience, or that one sect might obtain a pre-eminence, or two combined together, and establish a religion to which they would compel others to conform (Annals of Congress, Sat Aug 15th, 1789 pages 730 - 731).
p.s. James Madison and Thomas Jefferson were heavily involved in creating the constitution.