This is a tricky area. The question comes down to culpable or non culpable and when does one lose membership in the Church, because Baptism is the first necessity and True Faith being the second, the question ultimately can only be answered by God for any particular baptized person.
St Augustine expressed it his way:
St. Augustine: Sermon VIII, PL 46:838; SS vol.IV, p.254 ff.:
“Baptism does not profit a man outside unity with the Church, for many heretics also possess this Sacrament, but not the fruits of salvation. **Children baptized in other communions cease to be members of the Church when, after reaching the age of reason, they make formal profession of heresy, **as, for example, by receiving communion in a non-Catholic church.”
Pope Leo expressed the importance of True Faith :
POPE LEO XIII, ENCYCLICAL SATIS COGNITUM, JUNE 29, 1896, par. #9 :
“The Church, founded on these principles and mindful of her office, has done nothing with greater zeal and endeavour than she has displayed in guarding the integrity of the faith. Hence she regarded as rebels and expelled from the ranks of her children all who held beliefs on any point of doctrine different from her own. The Arians, the Montanists, the Novatians, the Quartodecimans, the Eutychians, did not certainly reject all Catholic doctrine: they abandoned only a certain portion of it. Still who does not know that they were declared heretics and banished from the bosom of the Church?
In like manner were condemned all authors of heretical tenets who followed them in subsequent ages. **‘There can be nothing more dangerous than those heretics who admit nearly the whole cycle of doctrine, and yet by one word, as with a drop of poison, infect the real and simple faith taught by our Lord and handed down by Apostolic tradition’ **(Auctor Tract. de Fide Orthodoxa contra Arianos)
“The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium. Epiphanius, Augustine, Theodore :, drew up a long list of the heresies of their times. St. Augustine notes that other heresies may spring up, to a single one of which, should any one give his assent, he is by the very fact cut off from Catholic unity. ‘No one who merely disbelieves in all (these heresies) can for that reason regard himself as a Catholic or call himself one. For there may be or may arise some other heresies, which are not set out in this work of ours, and, if any one holds to one single one of these he is not a Catholic’ (S. Augustinus, De Haeresibus, n. 88).
“…If then it be certain that anything is revealed by God, and this is not believed, then nothing whatever is believed by divine Faith: for what the Apostle St. James judges to be the effect of a moral delinquency, the same is to be said of an erroneous opinion in the matter of faith.
‘Whosoever shall offend in one point, is become guilty of all’ (Ep. James ii., 10).
Nay, it applies with greater force to an erroneous opinion. For it can be said with less truth that every law is violated by one who commits a single sin, since it may be that he only virtually despises the majesty of God the Legislator. But he who dissents even in one point from divinely revealed truth absolutely rejects all faith, since he thereby refuses to honour God as the supreme truth and the formal motive of faith. ‘In many things they are with me, in a few things not with me; but in those few things in which they are not with me the many things in which they are will not profit them’ ” (S. Augustinus in Psal. liv., n. 19).”
http://www.papalencyclicals.net/Leo13/l13satis.htm