Material heresy is heresy attributed to Protestants who are invincibly ignorant of Catholicism. According to Fr. Muller and Orestes Brownson, although they are not guilty of the sin of heresy, they are outside of the Church and can't be saved in that state. See extra ecclesiam nulla salus.
Fr. Muller wrote:
“Not guilty of the sin of heresy are all those who, without any fault of theirs, were brought up in a sect of Protestantism, and who never had an opportunity of knowing better. This class of Protestants are called invincibly or inculpably ignorant of the true religion, or material heretics..." 
"...material heretics may call them selves Christians, and their sects Christian Churches; but they are not the right sort of Christians and their sects are not the true Church of Christ. They are not Catholic Christians....
"...As long, then, as a material heretic, though through inculpable ignorance, adheres to an heretical sect, he is separated from Christ, because he is separated from his Body—the Catholic Church. In that state he cannot make any supernatural acts of divine faith, hope, and charity, which are necessary to obtain life everlasting, and therefore, if he dies in that state, he is pronounced infallibly lost by St. Augustine, St. Alphonsus and all the great Doctors of the Church." 
- ↑ Muller, Michael. The Catholic Dogma: Extra Ecclesiam Nullus Omnino Salvatur. Ch. 5 part 2, § 6. HAVING EXPLAINED WHAT CONSCIENCE IS, AND THE DIFFERENT KINDS OF CONSCIENCE, WE CAN NOW EASILY SHOW WHO ARE NOT GUILTY OF THE SIN OF HERESY.
- ↑ Muller, Michael. The Catholic Dogma: Extra Ecclesiam Nullus Omnino Salvatur. Invincible or inculpable ignorance
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