The website of the University of Dayton has a page dedicated to the dogmatic status and meaning of Mary's Perpetual Virginity. In it we find means by which the Church may have assurance of the infallibility of certain teachings:
There are other norms by which the Church may have assurance that a teaching has been infallibly revealed by God: consensus fidelium (i.e. general agreement among the entire body of believers "from the bishops down to the last of the lay faithful" [Lumen Gentium #12]); and "universal ordinary magisterium" (i.e. frequent authoritative teachings affirming one perspective on a topic given by the Pope alone, or by the episcopate in general).
Later on in the article we see this applied to the teaching of Mary's Perpetual Virginity:
On the topic of Mary's perpetual virginity, we have double assurance that the teaching may be considered as infallibly revealed in light of the statement of the fifth Ecumenical Council and by virtue of its constant use in the life of the Church afterwards (i.e. consensus of the faithful and universal ordinary magisterium).
So, one of the means by which the Church may be assured that the Perpetual Virginity of Mary is an infallible teaching is by virtue of it's constant use in the Church following it's statement in the 5th Ecumenical Council. However there are anathema's associated with denying the perpetual virginity of Mary which were announced, not only at that council, but in councils which followed.
The official acts of the 5th council contain an anathema condemning those who deny "that nativity of these latter days when the Word of God came down from the heavens and was made flesh of holy and glorious Mary, mother of God and ever-virgin, and was born from her.".
The Lateran Council of 649, convened by Pope Martin I contained this condemnation associated with denying the Perpetual Virginity:
If anyone does not, according to the Holy Fathers, confess truly and properly that holy Mary, ever virgin and immaculate, is Mother of God, since in this latter age she conceived in true reality without human seed from the Holy Spirit, God the Word Himself, who before the ages was born of God the Father, and gave birth to Him without corruption, her virginity remaining equally inviolate after the birth, let him be condemned.
A teaching is given to the Church, if you deny the teaching you are condemned, and then all the believers in the teaching are pointed to as proof of the infallibility of the teaching! What choice did they have when the only ones who get to stay in the Church are those who accept the teaching?
- It has been clarified to me elsewhere that a denier of a doctrine has separated themselves from the church rather than having been expelled but there appears to be no practical distinction as regards this question as they are, in either case, recognized by the Church as separated and their opinion is excluded from 'consensus fidelium'.
My question is, How can the universal acceptance of a teaching within the Roman Catholic Church stand as assurance of that teaching's infallibility when the teaching itself assigns condemnation to deniers of it and considers them to be separated from the Church and, therefore, removed from consensus fidelium?