"Clearly, millions of people have been exceedingly misled on that score."
More likely billions; in fact, virtually every Christian until some time after the Reformation and the overwhelming number of them to this day. Quix, do you believe that Christ was lying to the Apostles when he said that he would send the HS to guide The Church, or is it that the HS took 1600 years to finally get around to convincing a relative few Christian of the error of the previous centuries on the subject of the Theotokos?
There were always small groups here and there within and outside the Roman umbrella who had not bowed their knees to baal, so to speak. God has always had His own ways of protecting and preserving and nurturing a remnant.