heresy is a grave sin

Heresy is a sin because of its nature it is destructive of thevirtue of Christian faith. Its malice is to be measured therefore by the excellence of the good gift of which it deprives the soul. Now faith is the most precious possession of man, the root of his supernatural life, the pledge of his eternal salvation. Privation of faith is therefore the greatest evil, and deliberate rejection of faith is the greatest sin. St. Thomas (II-II, Q. x, a. 3) arrives at the same conclusion thus: "All sin is an aversion from God. Asin, therefore, is the greater the more it separates manfrom God. But infidelity does this more than any other sin, for the infidel (unbeliever) is without the true knowledge ofGod: his false knowledge does not bring him help, for what he opines is not God: manifestly, then, the sin of unbelief (infidelitas) is the greatest sin in the whole range of perversity." And he adds: "Although the Gentiles err in more things than the Jews, and although the Jews are farther removed from true faith than heretics, yet the unbelief of the Jews is a more grievous sin than that of theGentiles, because they corrupt the Gospel itself after having adopted and professed the same. . . . It is a more serious sin not to perform what one has promised than not to perform what one has not promised." It cannot be pleaded in attenuation of the guilt of heresy that heretics do not deny the faith which to them appears necessary tosalvation, but only such articles as they consider not to belong to the original deposit. In answer it suffices to remark that two of the most evident truths of thedepositum fidei are the unity of the Church and the institution of a teaching authority to maintain that unity. That unity exists in the Catholic Church, and is preserved by the function of her teaching body: these are two facts which anyone can verify for himself. In the constitution of the Church there is no room for private judgment sortingessentials from non-essentials: any such selection disturbs the unity, and challenges the Divine authority, of theChurch; it strikes at the very source of faith. The guilt ofheresy is measured not so much by its subject-matter as by its formal principle, which is the same in all heresies: revolt against a Divinely constituted authority.

http://www.newadvent.org/cathen/07256b.htm