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 I take the core of Tertium Quid’s argument (which I do not quote, follow the link) to be that an anti-sacramental theology does not adequately “remember the poor” and will have unfortunate historical consequences.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.)
in a post on a related blog, Tertium Quid remarks:
I became a Catholic because the Church is what comes back, but schismatic movements gradually move out of orthodoxy into heresy.
I became a Catholic because I wanted any children and grandchildren I might have to have something to come back to other than a denomination younger than granddad.
I became a Catholic when I realized that whenever the Church appears most corrupt and lost (beginning with Judas’ betrayal), grace through Christ himself and his saints will soon appear to seize victory from total defeat. The Church should have been defeated by the Jews in Jerusalem, the might Roman Emperors, the fall of Rome, the marauding of the Vikings, the scandal of the Great Schism, the despair of the Bubonic Plague, and the corruption of the Renaissance, not to speak of anything since 1517. The Church lives because God won’t let the all the keepers of the Real Presence die.
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see also Journey to Rome