One becomes a formal heretic by at least implicitly denying or rejecting the teaching authority of the Church, which is the formal motive of faith. Protestants do not accept the teaching authority of the Church. Consequently, they cannot be material heretics. Only Catholics can be material heretics. Material heresy is simply ignorance of fact ignorantia facti, i.e. not being aware of what the Church teaches yet accepting the teaching authority of the Church. So, for instance, when I was younger, I had a heretical understanding of the Immaculate Conception, but as soon as someone said, "But the Church teaches ..." I immediately accepted it because I accepted the teaching authority of the Church. Protestants reject out of hand the Church's teaching authority, so that even if you were to find a Protestant who happened to believe as the Church teaches on every single point of doctrine, he would still be a formal heretic for not accepting it on the authority of the Church.