Just about every established Church in 18th Century America was in some way formally connected to an orthodox Trinitarian creed except one -- the Quakers. The key Founders, like many of the population at large, didn't believe in the orthodox Trinitarian doctrines to which their churches adhered; rather they more often were church members out of social tradition (i.e., club membership). Some of the FFs truly hated creeds (even while being part of a church club that held to orthodox creeds). So the one type of Christianity that was closest to their unitarianism was Quakerism. That's one reason why they tended to love the Quakers despite the Quakers' refusal to take up arms. But as we will see there was one "key Founder" who didn't care for the Quakers at all.
First, George Washington:
Your principles and conduct are well known to me, and it is doing the People called Quakers no more than justice to say, that (except their declining to share with others the burthen of common defence) there is no denomination among us who are more exemplary and useful citizens.
Next, Thomas Jefferson:
But much I fear, that when this great truth shall be re-established, its votaries will fall into the fatal error of fabricating formulas of creed and confessions of faith, the engines which so soon destroyed the religion of Jesus, and made of Christendom a mere Aceldama; that they will give up morals for mysteries, and Jesus for Plato. How much wiser are the Quakers, who, agreeing in the fundamental doctrines of the gospel, schismatize about no mysteries, and, keeping within the pale of common sense, suffer no speculative differences of opinion, any more than of feature, to impair the love of their brethren. Be this the wisdom of Unitarians, this the holy mantle which shall cover within its charitable circumference all who believe in one God, and who love their neighbor! I conclude my sermon with sincere assurances of my friendly esteem and respect.
Next, James Madison. Well he married a Quaker.
But the Quaker love fest ends with John Adams. Here are some nasty remarks of his towards them:
You will see by the Papers inclosed, that We have been obliged to attempt to humble the Pride of some Jesuits who call themselves Quakers, but who love Money and Land better than Liberty or Religion. The Hypocrites are endeavouring to raise the Cry of Persecution, and to give this Matter a religious Turn, but they cant succeed. The World knows them and their Communications. Actuated by a land jobbing Spirit, like that of William Penn, they have been soliciting Grants of immense Regions of Land on the Ohio. American Independence has disappointed them, which makes them hate it. Yet the Dastards dare not avow their Hatred to it, it seems.