I agree with St. Thomas and the Catholic Encyclopedia:
HERESY
I. CONNOTATION AND DEFINITION
St. Thomas (II-II:11:1) defines heresy:
“a species of infidelity in men who, having professed the faith of Christ, corrupt its dogmas”. "The right Christian faith consists in giving one’s voluntary assent to Christ in all that truly belongs to His teaching. There are, therefore, two ways of deviating from Christianity: the one by refusing to believe in Christ Himself, which is the way of infidelity, common to Pagans and Jews; the other by
restricting belief to certain points of Christ’s doctrine selected and fashioned at pleasure, which is the way of heretics. The subject-matter of both faith and heresy is, therefore, the deposit of the faith, that is, the sum total of truths revealed in Scripture and Tradition as proposed to our belief by the Church. The believer accepts the whole deposit as proposed by the Church; the heretic accepts only such parts of it as commend themselves to his own approval. The heretical tenets may be ignorance of the true creed, erroneous judgment, imperfect apprehension and comprehension of dogmas: in none of these does the will play an appreciable part, wherefore one of the necessary conditions of sinfulness–free choice–is wanting and such heresy is merely objective, or material. On the other hand the will may freely incline the intellect to adhere to tenets declared false by the Divine teaching authority of the Church. The impelling motives are many: intellectual pride or exaggerated reliance on one’s own insight; the illusions of religious zeal; the allurements of political or ecclesiastical power; the ties of material interests and personal status; and perhaps others more dishonourable.
Heresy thus willed is imputable to the subject and carries with it a varying degree of guilt; it is called formal, because to the material error it adds the informative element of “freely willed”.
IV. GRAVITY OF THE SIN OF HERESY
Heresy is a sin because of its nature it is destructive of the virtue 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. A sin, therefore, is the greater the more it separates man from God.* But infidelity does this more than any other sin, for the infidel (unbeliever) is without the true knowledge of God: 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 the Gentiles, 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 to salvation, 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 the depositum 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 sorting essentials from non-essentials***: any such selection disturbs the unity,and challenges the Divine authority, of the Church; it strikes at the very source of faith.*** The guilt of heresy 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.
(The Catholic Encyclopedia, Volume VII. Published 1910. New York: Robert Appleton Company. Nihil Obstat, June 1, 1910. Remy Lafort, S.T.D., Censor. Imprimatur. +John Cardinal Farley, Archbishop of New York)
newadvent.org/cathen/07256b.htm
In Summation, I reiterate, heretics and are not Christian in the slightest, that includes but is not limited to, many Christian denominations. GOD Bless you.