Correction:
I wrote --
It is not possible for the Church to allow a heresy to be taught for five hundred years, sorry.
I should have written --
It is not possible for most if not all of the theologians of the Church to teach a heresy over three hundred years.
I have to change that, because of course, the Immaculate Conception was denied by some for a while, including -- arguably -- St. Thomas, and more shockingly, St. Bernard, who, as legend has it, was breast-fed by Mary with the milk of wisdom itself ( no, I didn't make this up, look up Lactation of St. Bernard ).
But just as with the Immaculate Conception, where over time more and more people believed in it until basically everyone believed, and then it became a dogma, the idea of some kind of implicit faith being sufficient for salvation is on the same track. The trend, long before Vatican II, was for more and more theologians to accept that some form of implicit faith being sufficient for salvation is possible.
It's true that very few agree on what must be believed implictly, and what must be believed explicitly. This may be the "vortex of confusion" that Augustine feared. What he didn't see, I think, and what perhaps God showed me after much agonizing over this question -- and I do mean agonizing -- is that it's only confusing if you think that you HAVE to know every last secret of the mind of God. If you leave it a mystery, then there is no problem. God can save certain people under certain conditions, but we don't always know what those conditions are, and therefore, they should take steps to learn the faith and be baptized...
The following quote from an encylical of Pius IX, to me, is the last word on the matter. It explains that the exact boundaries of invincible ignorance are a MYSTERY at which our minds should stop, which we should not try to penetrate further.
"For, it must be held by faith that outside the Apostolic Roman Church, no one can be saved; that this is the only ark of salvation; that he who shall not have entered therein will perish in the flood; but, on the other hand, it is necessary to hold for certain that they who labor in ignorance of the true religion, if this ignorance is invincible, are not stained by any guilt in this matter in the eyes of God. Now, in truth, who would arrogate so much to himself as to mark the limits of such an ignorance, because of the nature and variety of peoples, regions, innate dispositions, and of so many other things? For, in truth, when released from these corporeal chains "we shall see God as He is" [ 1 John 3:2], we shall understand perfectly by how close and beautiful a bond divine mercy and justice are united; but, as long as we are on earth, weighed down by this mortal mass which blunts the soul, let us hold most firmly that, in accordance with Catholic teaching, there is "one God, one faith, one baptism" [ Eph. 4:5 ]; it is unlawful to proceed further in inquiry.
Of course, this didn't stop all the theologians from inquiring, or myself, but maybe it should.