Sparks has written a number of interesting works on Unitarianism's history, most of them available in their entirely on googlebooks. In this one Sparks discusses the early British Unitarians who just happened to be some of the most notable philosophical influences on the American Founders. Sparks reproduces a number of letters he exchanged with his Trinitarian critics, indeed critics who stress the point that Unitarians are not Christians, and notes if that's so, then lets look at the list of men who aren't "Christians." In one passage Sparks states:
Your sweeping denunciation embraces all Unitarians of every age and country. If your charges are well-founded, Newton, Locke, and Chillingworth, were "no christians in any correct sense of the word, nor any more in the way of salvation, than Mohammedans or Jews?"
Sparks was adamant that Locke was indeed a Unitarian. Again, replying to a Trinitarian critic:
And Locke must still be considered a Unitarian, till he can be proved a Trinitarian ; a task, which it is not likely you will soon undertake. At all events, he had no faith in the assemblage of articles, which you denominate the essence of christianity, and without believing which, you say, no one can be called a Christian. His whole treatise on the Reasonableness of Christianity bears witness to this truth. For the leading object of that work is to show, that "the Gospel was written to induce men into a belief of this proposition, 'that Jesus of Nazareth was the Messiah,' which if they believed, they should have life."* He says nothing about total depravity, the atonement, the "sanctifying spirit of an Almighty Surety," nor any of your peculiar doctrines. Yet who has done more to elucidate the sacred Scriptures, or to prove the consistency and reasonableness of the religion of Jesus? Your rule, however, will take from him the Christian name.
Sparks notes how Unitarians often held high positions as Anglican Divines:
Those ornaments of the Episcopal church, Dr. Samuel Clarke, Hoadly, Law, and Blackburne, must be ranked with those, among whom "we look in vain for the monuments of the reforming and purifying power" of their faith.
Sparks then rattles off a whole slew of names of prominent theologians who believed in the unitarian heresy:
Envlyn, Whiston, Priestley, Lindsey, Price, Jebb, Wakefield, Chandler, Taylor, Benson, Cappe, Foster, Kippis, and a host of others among the English Unitarians,...
Along with the better known Locke and Newton, Joseph Priestley, Richard Price, Benjamin Hoadly, and Samuel Clarke were some of the most important British Whig thinkers who influenced America's Founders. When asked to speak on the attributes of God, James Madison turned to Samuel Clarke (not John Witherspoon!) for theological authority. Here is how the Stanford Encyclopedia of philosophy describes Samuel Clarke and his "controversy":
In 1712, apparently against the advice of some of Queen Anne's ministers, Clarke published The Scripture Doctrine of the Trinity, which was accused of Arianism, the view that Christ is divine but created. The ensuing controversy culminated two years later in his humiliating promise to the Upper House of Convocation not to preach or write on the topic any longer. However, this act of submission did not silence the correct rumors that he, like Newton himself, was still an Arian.
The entry goes on to describe what the "orthodox" thought of Clarke's opinions:
How much these suspicions of heterodoxy damaged his ecclesiastical career is unclear. However, Voltaire reports that Bishop Gibson effectively prevented Clarke's elevation to the see of Canterbury by pointing out that Clarke was indeed the most learned and honest man in the kingdom, but had one defect: he was not a Christian.
Again, whether Unitarians like Sparks, Locke, Clarke, America's key Founders, et al. are entitled to the "label" Christian is a matter of debate. But when debating Christian America apologists, it's important to point out to them that their theology doesn't consider these men to be "Christians," but rather "heretics" at best, "infidels" at worst.