James Madison believed the United States was a Christian Nation.

The words of James Madison while forming the nation is the greatest proof of our Christian heritage. His words while forming the nation reflect Christian beliefs, rather than the words of a syncretist ex post facto.

How could Madison believe two different religions joined together could form a National Religion in a Christian Nation?

MR. MADISON thought, if the word 'National' was inserted before religion, it would satisfy the minds of honorable gentlemen. He believed that the people feared 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 1:434-435) Saturday August 15, 1789

A national religion cannot refer to every religion, it must refer to a specific religion, that being Christianity. Madison's personal beliefs about Christianity aren't as important as his believing we were a Christian nation.

"If the public homage of a people can ever be worthy the favorable regard of the Holy and Omniscient Being to whom it is addressed, it must be that in which those who join in it are guided only by their free choice, by the impulse of their hearts and the dictates of their consciences; and such a spectacle must be interesting to all Christian nations as proving that religion, that gift of Heaven for the good of man, freed from all coercive edicts, from that unhallowed connection with the powers of this world which corrupts religion into an instrument or an usurper of the policy of the state...Upon these principles and with these views the good people of the United States are invited, in conformity with the resolution aforesaid, to dedicate the day above named to the religious solemnities therein recommended."
Given at Washington, this 23d day of July, A. D. 1813.[seal.] JAMES MADISON

Here, Madison claims "these principles" are proper for Christian nations to conform to. Freedom of Conscience is what he is referring to, while comparing us with all Christian nations, to what common sense dictates, we must be a Christian nation.

Common sense asks why would Madison compare us with a Christian nation if he did not believe we were such?

James Madison believed the context of the First Amendment, as the other Founding Fathers believed, that freedom of conscience was granted to all, and a Christian denomination was the only sect that could not be established by the Federal Government. Notice the correct context for the First Amendment by the Father of the Bill of Rights:

"[A]ll men have an equal, natural and unalienable right to the free exercise of religion, according to the dictates of conscience; and that no particular sect or society of Christians ought to be favored or established by law in preference to others."
Col. George Mason
Kate Mason Rowland, The Life of George Mason (New York: G. P. Putnam's Sons, 1892, Vol. I, p.28.

James Madison agreed with this context as did the other framers. If Madison was a syncretist at this time, it's a mystery why he would tell Christians we were a Christian nation. It's more likely he was confused about Christianity, his relationship with the infidel Thomas Jefferson as evidence. The other religions were not involved in this establishment, and never had a chance to be the established church:

Thomas Jefferson also believed the First Amendment only referred to a denomination of Christianity

"[T]he clause of the Constitution which, while it secured the freedom of the press, covered also the freedom of religion, had given to the clergy a very favorite hope of obtaining an establishment of a particular form of Christianity through the United States; and as every sect believes its own form the true one, every one perhaps hoped for his own, but especially the Episcopalians and Congregationalists." Thomas Jefferson to Benjamin Rush on September 23, 1800.

Jefferson would never have neglected the other religions if the amendment encompassed them, for it was against his sense of duty:

"On every question of construction, carry ourselves back to the time when the Constitution was adopted, recollect the spirit manifested in the debates, and instead of trying what meaning may be squeezed out of the text, or invented against it, conform to the probable one in which it was passed." Thomas Jefferson to Supreme Court Justice William Johnson June 12, 1823

The ratifiers make it clear, the First Amendment granted freedom of conscience but the establishment clause only referred to a particular sect of Christianity., as Constitution signer Henry Abbot explains:

"Many wish to know what religion shall be established. I believe a majority of the community are Presbyterians. I am, for my part, against any exclusive establishment; but if there were any, I would prefer the Episcopal."Elliot's Debates, Vol. IV, pp. 191-192, July 30, 1788.

Here also is First Amendment Ratifier Samuel Johnston in the North Carolina ratifying convention:

I know but two or three States where there is the least chance of establishing any particular religion. The people of Massachusetts and Connecticut are mostly Presbyterians. In every other State, the people are divided into a great number of sects. In Rhode Island, the tenets of the Baptists, I believe prevail...I hope, therefore, that gentlemen will see there is no cause of fear that any one religion shall be exclusively established. July 30, 1788

The Christian nation naysayers have James Madison to contend with.