In Catholicism, why is heresy a mortal sin?

According to Catholicism, an act of heresy is a mortal sin. > "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same ... Catechism of the Catholic Church #2089 In particular, dogmatic statements end with warnings that not accepting the dogma results in falling away from the Catholic faith. For example, > Hence if anyone, which God forbid, should dare willfully to deny or to call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic Faith. Munificentissimus Deus #45 Why is this the case? Why do matters of doctrine result in loss of salvation?