A "True Church" means the Church originally founded by God Himself (Jesus Christ) and containing the fullness of the Truth as delivered by Him through his Apostles.
Heresy: (a Roman Catholic source, but the basic definition is the same)
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".
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Heresy differs from apostasy. The apostate a fide abandons wholly the faith of Christ either by embracing Judaism, Islamism, Paganism, or simply by falling into naturalism and complete neglect of religion; the heretic always retains faith in Christ. Heresy also differs from schism. Schismatics, says St. Thomas, in the strict sense, are they who of their own will and intention separate themselves from the unity of the Church....
http://www.newadvent.org/cathen/07256b.htm
