The Founders on Original Sin & Human Nature:

America's Key Founders (you know them, Washington, Adams, Jefferson, Madison, Franklin, et al.) either outright rejected original sin, or if they believed in it at all had a more positive, Arminian view of human nature, and rejected Calvin's total depravity.

First, Thomas Jefferson, in his October 31, 1819 letter to William Short, listed original sin with every other tenet of orthodox Christianity as things he rejected. Quoting him:

* e. g. The immaculate conception of Jesus, his deification, the creation of the world by him, his miraculous powers, his resurrection and visible ascension, his corporeal presence in the Eucharist, the Trinity; original sin, atonement, regeneration, election, orders of Hierarchy, &c.

Franklin rejected original sin in his 1735 A Defense of Mr. Hemphill’s Observations:

But lest they shou’d imagine that one of their strongest Objections hinted at here, and elsewhere, is designedly overlook’d, as being unanswerable, viz. our lost and undone State by Nature, as it is commonly call’d, proceeding undoubtedly from the Imputation of old Father Adam’s first Guilt. To this I answer once for all, that I look upon this Opinion every whit as ridiculous as that of Imputed Righteousness. ’Tis a Notion invented, a Bugbear set up by Priests (whether Popish or Presbyterian I know not) to fright and scare an unthinking Populace out of their Senses, and inspire them with Terror, to answer the little selfish Ends of the Inventors and Propagators. ’Tis absurd in it self, and therefore cannot be father’d upon the Christian Religion as deliver’d in the Gospel. Moral Guilt is so personal a Thing, that it cannot possibly in the Nature of Things be transferr’d from one Man to Myriads of others, that were no way accessary to it. And to suppose a Man liable to Punishment upon account of the Guilt of another, is unreasonable; and actually to punish him for it, is unjust and cruel.

John Adams likewise rejected original sin:

The origin of mal moral [moral evil -- Ed.] is liberty, the self determining power of free agents, endowed with reason & conscience & consequently accountable for their conduct....I have read the Holy Fathers of the Hindus, of the disciples of Pythagoras of Frederick of Prussia of Soame Jenings of Dr. Edwards & many others and am no more satisfied than with Eve's apple. I have no difficulty about it. I am answerable for my own sins because I know they were my own fault; and that is enough for me to know.

-- John Adams to Francis van der Kemp, February 23, 1815. Taken from James H. Hutson, The Founders on Religion, pp. 200-01.

James Madison, in Federalist 10, did not, as some argue, endorse the notion of Calvinist total depravity, but rather Arminian partial depravity, believing man's nature capable of great good or great evil. As he wrote:

As there is a degree of depravity in mankind which requires a certain degree of circumspection and distrust: So there are other qualities in human nature, which justify a certain portion of esteem and confidence. Republican government presupposes the existence of these qualities in a higher degree than any other form.

Alexander Hamilton, whom some have termed the Rousseau of the right, sounds positively humanistic in Federalist 22 when he notes the people not God form the solid basis of America's Constitution:

The fabric of American Empire ought to rest on the solid basis of THE CONSENT OF THE PEOPLE. The streams of national power ought to flow immediately from that pure original fountain of all legitimate authority.

This neatly comports with Rousseau's notion of "the general will." Hamilton's confidence in humanity is also decidedly anti-Calvinistic, and, if Christian at all, clearly Arminian influenced. As George Willis Cooke aptly noted:

The doctrine of degrees, as taught by the Calvinists, was the spiritual side of the assertion of the divine right of kings. On the other hand, when the people claim the right to rule, they modify their theology into Arminianism. From an age of the absolute rule of the king comes the doctrine of human depravity; and with the establishment of democracy appears the doctrine of man's moral capacity.

Though some Arminians remained Trinitarian, they had a tendency to "slip" into unitarianism and rationalism. Indeed when studying Founding era literature one frequently sees unitarian rationalists like Jonathan Mayhew also referred to as "Arminians."

Barry Shain refers to these Arminian/unitarian/rationalist types as "Christian humanists," because they often presented their ideas under the auspices of "Christianity." Notice how Franklin's above quoted passages argues true Christianity rejects original sin! Most evangelicals and Catholics, however, would argue, like Mormonism, this isn't Christianity, regardless of what it terms itself. Dr. Gregg Frazer, an evangelical, terms this belief system of America's key Founders not Christianity but theistic rationalism. In other words if that these "Christian humanists" rejected nearly every single tenet of traditional Christianity (see Jefferson's above quoted remarks) doesn't separate them from "Christianity" then what does?