The most radical departure from history in the councils teachings concerned religious freedom.
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George Sim Johnston's Crisis Magazine apologia pro Vatican II. Although I certainly contend many of his assertions and disagree with most of his conclusions (and his naive optimism, even now, is downright amusing), this is about as reasonable and intelligent of an argument defending the Second Vatican Council as one will find
I personally am a big fan of George Sim Johnston, as he's the most honest conservative I've ever read. While many conservatives will complain and contest even the most obvious observations of traditionalists as to what the Council changed (asserting that the changes are merely figments of our imagination), Johnston freely admits, defends and celebrates those changes (ala Yves Congar). Dialogue is much more productive when the participants are not having to waste time arguing about basic facts and definitions, and get to the heart of the matter - the results and effects of said changes.
I shall have to investigate more of this man's writing.