Louis Card. Billot, Tractatus de Ecclesia Christi (Romae, 1927), v. 1, p. 296-298.
Billot: Thesis XI. "Although the character of baptism is sufficient of itself to incorporate a man into the true Catholic Church, nevertheless it requires in adults a twofold condition for this effect. The first condition is that the social bond of unity of faith not be impeded by formal or even merely material heresy. Nevertheless, because this impediment is brought in only by that heresy which passes into open profession, it must be said that only notorious heretics are excluded from the body of the Church.
"Now, heretics are divided into formal and material. Formal heretics are those to whom the authority of the Church is sufficiently known; material, those who labor under invincible ignorance concerning the Church herself, and choose in good faith another rule for their guide. Heresy therefore is not imputed to material heretics as sin, nor, furthermore, is there necessarily a lack of that supernatural faith which is the beginning and root of all justification. For perhaps they explicitly believe the principal articles, and believe the rest not explicitly but implicitly, by the disposition of mind and the good will of adhering to all those things which would be sufficiently proposed to them as revealed by God. Furthermore, they can still belong in voto to the body of the Church, and have the other conditions required for salvation. Nevertheless, so far as pertains to the real incorporation in the visible Church of Christ presently being treated, the thesis places no distinction between formal or material heretics, understanding everything according to the notion of materal heresy just explained, which is also the only proper and genuine notion. For if by a material heretic you meant one who, professing that in matters of faith he depends on the Magisterium of the Church, but still denies something defined by the Church which he does not know has been defined, or holds an opinion opposed to Catholic doctrine for the reason that he thinks that it is taught by the Church, it would in this case be absurd to posit that material heretics are outside the body of the true Church, and in addition, in this way, the legitimate meaning of the word would be completely overturned. For only then is it said that there is material sin, when the things that belong to the definition of such a sin are materially posited, but excluding reflection or deliberate volition. Now, what pertains to the definition of heresy is the departure from the rule of the ecclesiastical Magisterium, which in this case is not present, because it is a simple error of fact concerning that which the rule dictates. And therefore, there can be no place even materially for heresy."