Thomas Jefferson (1743-1826) was the principle author of the Declaration of Independence, the third President of the United States, and a primary architect of the American tradition of separation of church and state. 

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Believing that religion is a matter which lies solely between man and his God, that he owes account to none other for his faith or his worship, that the legislative powers of government reach actions only, and not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their Legislature should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between Church and State (Letter to the Danbury Baptists, 1802).

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The clergy, by getting themselves established by law and ingrafted into the machine of government, have been a very formidable engine against the civil and religious rights of man (Letter to J. Moor, 1800).

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History, I believe, furnishes no example of a priest-ridden people maintaining a free civil government. This marks the lowest grade of ignorance of which their civil as well as religious leaders will always avail themselves for their own purposes (Letter to von Humboldt, 1813).

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In every country and in every age, the priest has been hostile to liberty. He is always in alliance with the despot, abetting his abuses in return for protection to his own (Letter to H. Spafford, 1814).

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James Madison (1751-1836) is popularly known as the "Father of the Constitution." More than any other framer he is responsible for the content and form of the First Amendment. His understanding of federalism is the theoretical basis of our Constitution. He served as President of the United States between 1809-1817.  

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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).

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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).

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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).

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No state shall infringe the equal rights of conscience, nor the freedom of speech, or of the press, nor of the right of trial by jury in criminal cases [Proposed amendment to make certain parts of the Bill of Rights to apply to the states].

The Congressional Record of August 17, 1789 made the following comment on Madison's proposal:

MR. MADISON Conceived this to be the most valuable amendment on the whole list; if there was any reason to restrain the government of the United States from infringing upon these essential rights, it was equally necessary that they should be secured against the state governments; he thought that if they provided against the one, it was an necessary to provide against the other, and was satisfied that it would be equally grateful to the people (from Alley, James Madison on Religious Liberty, pp. 75-76).

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George Mason (1725-1792). Few men had more influence on the shaping of our Constitution.  As a member of Constitutional Convention, Mason was an outspoken advocate for federalism and limited government. As a member of the Virginia ratifying convention, he proposed a set of amendments that later served as a model for our Bill of Rights.

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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, and therefore all Men have an equal natural and unalienable Right to the free Exercise of Religion, according to the Dictates of Conscience, and that no particular religious Sect or Society ought to be favored or established by Law, in Preference to others (Robert A. Rutland, The Papers of George Mason, Vol. 3, p. 1071, 1119 [footnote].

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...it is contrary to the principles of reason and justice that any should be compelled to contribute to the maintenance of a church with which their consciences will not permit them to join, and from which they can derive no benefit; for remedy whereof, and that equal liberty as well religious as civil, may be universally extended to all the good people of this commonwealth (Pamela Copeland and Richard MacMaster, The Five George Masons, pp. 176).

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