Regarding Newman’s claim that Calvinism tends, historically, to slide into Unitarianism, a post by Tertium Quid connects this with our sensory human nature:
….Thus, if you look at the history of Calvinism in several countries, it follows a pattern: zeal for the written Word of God, attempted conversion of life and practice to scriptural standards, renunciation of sacraments and tradition, open rebellion against all bishops, predestination to the nth degree, vigorous debate about the meaning of scriptures, debilitating debate about church governance, debate re how to measure the regeneracy of “cradle Christians,” weakening of the 3rd and 4th generations, development of “Calvinism-light”, e.g., Harvard in the 18th century and Yale in the 19th, schisms, revivals, and increased focus on the individual as the rational discerner of the truths of God….
Certainly arguments about historic trends are fraught with difficulties and call for charity on all sides. I take the core of Tertium Quid’s argument (which I do not quote, follow the link) to be that an anti-sacramental theology does not adequately “remember the poor” and will have unfortunate historical consequences.
(By the way, Edward Banfield quipped somewhere that a Unitarian was a lapsed Christian and that he was a lapsed Unitarian.)