Perhaps the primary error — because it comes so close to taking the heresy of Modernism and making it into a dogma of the new religion — is that of Religious Liberty. Traditionally, the Church holds that since It alone teaches the truth, all other religions represent a dangerous compromise with error. It forces no one to become a Catholic (you can’t control someone’s mind), but reserves the right to keep non-believers from spreading their errors and from publicly acting in accord with an incorrect moral code. (The discussion assumes that the Church is in a political position to say or do something about such matters.) To a modern American this sounds like a rerun of the Spanish Inquisition, but a little reflection will remind you that virtually all of the nations of the world functioned in this manner until very recently. Even here in these United States we had laws which regulated immoral acts like contraception, divorce, sodomy, abortion, and suicide — only in the past fifty years or so have they been eliminated or greatly liberalized.
Now you might, correctly, point out that even today the Church strongly disapproves of the immoral acts listed immediately above. In fact It does, but you will find a contradictory statement in Dignitatis humanae, the Vatican II declaration on Religious Liberty:
This Vatican Synod declares that the human person has a right to religious freedom. This freedom means that all men are to be immune from coercion on the part of individuals or of social groups and of any human power, in such wise that in matters religious no one is to be forced to act in a manner contrary to his own beliefs. Nor is anyone to be restrained from acting in accordance with his own beliefs, whether privately or publicly, whether alone or in association with others, within due limits. (emphasis added) 
Now, the phrase “within due limits” and other statements that hold that the State may protect itself and its citizens from harm perpetrated in the name of religion might seem to make this statement harmless enough — but who defines the “due limits” in a society with religious liberty? certainly not the Catholic Church nor any other Christian body. At best the “due limits” might be determined by the Gallop Poll, but in practice such limits are usually determined by politicians, lobbyists, bankers, and lawyers. (e.g. Kennedy, Cuomo, Rockefeller, Earl Warren, etc.) And guess who decides which religions are or are not harmful to the public?
In Sacred Scripture the psalmist teaches that Christ is to be regarded as King in a literal manner:
The Lord said to Me: “Thou art My Son; this day I have begotten Thee. Ask of Me and I will give Thee the nations for an inheritance, and the ends of the earth for Thy possession. Thou shalt rule them with an iron rod; Thou shalt shatter them like an earthen dish.” And now, O kings, give heed; take warning, you rulers of the earth. Serve the Lord with fear, and rejoice before Him; with trembling pay homage to Him.
Pope Pius XI in establishing the liturgical feast of Christ the King tells us that:
… it is of the Catholic Faith to believe that Jesus Christ has been given to men as Redeemer in whom we are to believe, and as a Lawgiver whom we are to obey…. Anyone would err gravely, on the other hand, who would take away from Christ as man the rule over civil affairs, since He has been given by the Father such complete power over created things that all are subject to His will.
By the mid 1970s, the few remaining Catholic countries in the world amended their constitutions to conform to Vatican II and ceased being officially Catholic. Even the Vatican concordat with Italy was amended to reflect a change in the spiritual status previously enjoyed by the City of Rome. This must have been particularly difficult in South American countries like Ecuador, previously a republic dedicated in its Constitution to the Sacred Heart of Jesus; or Argentina, where the Blessed Virgin Mary was legally Commander in Chief of the Armed Forces. (Guess which formerly Catholic continent is rapidly becoming Protestant, and which Armed Force got “creamed” by Queen Elizabeth’s troops after losing their Commander in Chief.)