Is material heresy venial sin?
Material heresy is not always a sin. It can be mortally sinful, venially sinful, and without sin depending on what kind of ignorance someone has.
The following is an excerpt from
Teologia Moral En Cuadros (AD 1862) and its English translation by Google:
"Puesto que la obstinacion hace al hereje, no lo es el que por ignorancia sostiene un error contrario a la fe, sin querer persistir en el contra las decisiones de la Iglesia. Esta es la que se denomina herejia material; es sin pecado si la ignorancia es invencible; y culpable si la ignorancia es vencible, en diversos grados segun la culpabilidad de la ignorancia; mas no hace incurrir en las penas impuestas á los herejes, y todo confesor puede absolver de ella; mientras que la herejía formal queda reservada como se dirá mas adelante."
"Since obstinacy makes the heretic, it is not the one who out of ignorance maintains an error contrary to the faith, without wanting to persist against the decisions of the Church. This is what is called material heresy; it is without sin if ignorance is invincible; and guilty if ignorance is beatable, in varying degrees according to the guilt of ignorance; but it does not cause the penalties imposed on heretics to be incurred, and every confessor can absolve from it; while the formal heresy is reserved as will be said later."