Jared Sparks on Locke's Arianism:

I regularly check googlebooks for new additions of obscure works which heretofore you'd never be able to access unless you traveled to the libraries (for instance Harvard's or Princeton's) where they are located. Alas, works that aren't in the public domain are only previewed; but there are many great originals from hundreds of years ago which have been digitized in full.

Most scholars who have looked into the matter conclude that Locke, like Milton, Newton, Clarke and many other British divines, was an Arian heretic. One reason why scholarly "work" has to be done to conclude this is (many people don't realize) before the Enlightenment, religious heretics didn't have the right to speak their mind and could be legally punished at worst executed (like Servetus) for publicly proclaiming their heresy. It's not as though Locke could just announce to the world "I don't believe in the Trinity"; he would be executed as was Servetus. Indeed, one prime reason why these enlightened philosophers did their epistemological work to bring freedom of conscience to Western thought was so folks like them could be open heretics.

Jared Sparks, distinguished biographer of George Washington and other Founders, President of Harvard, and a notable Unitarian theologian of the Founding era, wrote a book, published in 1823, comparing Unitarian and Trinitarian theology. On pp. 375-83, He convincingly argues for Locke's Arianism.