People not being able to necessarily comprehend or even know all the dogmas of the faith does not mean they do not hold the faith. The medieval canonists dealt with this very question. They determined that since this was usually not possible in the bulk of humanity, what was required of them was to believe that God exists and to do good works, and to have the general will to believe what the hierarchy told them to and to profess unity with the Church. This “implicit faith” derived from an explicit faith in the Church was all that was sufficient. Pope Innocent IV (an important canonist of that time in his own right) said that one who errs in matters of dogma has the faith “so long as he believes it is the faith of the Church. In that case, the faith of the Church replaces his opinion, though his opinion is false, it is not his faith, but his faith is the faith of the Church." (Innovent IV, Commentaria in quinque libros decretalia, Ad liber I).
This is all the more the case since language can change over time. That’s why Cardinal Manning, in his work on the Temporal Mission of the Holy Ghost said:
Cardinal Manning Wrote:The enunciation of the faith by the living Church of this hour, is the maximum of evidence, both natural and supernatural, as to the fact and the contents of the original revelation.
And again:
Cardinal Manning Wrote:The maturity of theology is not antiquity, but its later days ; and language which was blameless in earlier and simpler times, may become heterodox in after ages : for example, the procession of the Holy Ghost from the Father through the Son, the Immaculate Nativity of the Mother of God, and the like. Again, language which once was heterodox may become the test of truth, as the Homoousion, which was condemned by the Council of Antioch in the Sabellian sense, and in half a century was inserted in the Creed by the Council of Nicea. No critic except the living and lineal judge and discerner of truth, the only Church of God, can solve these inequalities and anomalies in the history of doctrine. To the Church the facts of antiquity are transparent in the light of its perpetual consciousness of the original revelation.
Furthermore, the heresy necessary for one to be therefore ipso facto separated from the Church without anything else had to be of such a nature that it was beyond dispute and recognizable to all. One way was making a profession of faith in another society other than the Catholic Church (like, say, formally converting to the Nestorian or Methodist Church, etc.). That is an obvious thing that all can recognize.
The other was through “refusing the infallible teaching authority.” This would mean explicitly rejecting the judgment of the Church. Even simply stating a heretical proposition does not also mean one is refusing the infallible teaching authority. More context is needed to show one is manifestly rejecting that authority and not docile to it, but erring in good faith (in which case the faith of the Church covers his false opinion).
The main problem is with those who declare the theological deductions of others to be heretical, and therefore consider them outside the Church. In such a case, the one accused publically professes to believe all the Church has taught and to accept all the infallible judgments of the Church, and deduces doctrines he believes to be consonant with those judgments. The other person disagrees that those deductions are consonant. This is not enough for the latter person to believe the former has separated himself from the visible society of the Church.
Take the example Southpaw gives above: “men have the liberty to choose and follow any religion they desire.” First of all, this in itself is ambiguous and may or may not go against the infallible judgment of the Church. If he means that man has no obligation to receive the true revelation of God in faith and to firmly embrace it and follow it, then it does go against that judgment (more context is needed to show one is manifestly and publicly rejecting the infallible authority and not docile to it, but erring in good faith). However, if it means as the Church has proposed (which I think he is actually alluding to), that man must be free from the constraints of the state in his religious activity, so long as his actions are not harmful to the common good or contrary to the natural law, then there is no defintiive condemnation of that specific proposition that I know of. It’s a deduction that Southpaw believes is not consonant with past condemnations, but which others argue is consonant.
If this kind of disagreement was sufficient for one person to declare another no longer a member of the Church, the work of theologians would disintegrate the Church—not to mention if lay people did so too! Disagreements between Catholic theologians as to whether certain propositions were true or heretical have existed as long as theologians have existed. It is only once the Church provides the definitive judgment on such disputed points does one run the risk of being separated from the visible society, that is the Church.
This is why the medieval canonists, when determining when this kind of ipso facto falling out the Church happened, wisely excluded what they called “new heresies.” If they had included “new heresies,” it would leave it up to the individual apart from the authoritative judgment of the Church to pass sentence on doctrines and therefore to judge who was and was not a member of the Church. As such, a “new heresy” must first be judged by the Church to actually be a heresy.
In addition, people may also disagree as to the meaning or even scope of past judgments—they may accept the judgment as they understand the meaning and scope, and disagree with someone else’s understanding—while both are open and docile to the further judgment of the Church on the matter. This also is not sufficient for one to be ipso facto outside the Church.
(the above is not meant to get into the situation where the Magisterium is one of the parties in the “dispute.” The acceptance and promulgation of one deduction over another by the Magisterium, adds more weight to that deduction and more of an obligation toward assent to it, depending on how it has been promulgated).