heretic

found in comment 5 here http://www.calledtocommunion.com/2009/06/christ-founded-a-visible-church/#why

The word ‘member’ is used in different senses, and that creates the ambiguity to which you are referring regarding membership. So, let’s define some terms.

By ‘heresy’ here we are speaking of formal heresy, as it is defined in the Catechism:

“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.” (CCC 2089)

This is the definition used in Canon Law (see Can. 791).

Notice the repeated word ‘obstinate’. This involves a person who isn’t merely accidentally or unknowingly denying something that the Church teaches must be believed with divine and catholic faith. (Doing so unknowingly or without an awareness or understanding that the Church taught otherwise, would be material heresy.) In a case of formal heresy, the person is told clearly what the Church teaches must be believed with divine and catholic faith (the phrase ‘divine and catholic faith’ is a technical term, and refers to that which requires the highest level of assent — see Canon 750 in the Code of Canon Law), and he obstinately denies it or obstinately doubts it.

Formal heresy incurs automatic (latae sententiae) excommunication:

Can. 1364 §1. … [A]n apostate from the faith, a heretic, or a schismatic incurs a latae sententiae excommunication.

The word ‘heresy’ there is being used as it was defined in Canon 791. In other words, it is referring to formal heresy, not material heresy. In light of that, now consider what Pope Pius XII says in Mystici Corporis Christi:

Actually only those are to be included as members of the Church who have been baptized and profess the true faith, and who have not been so unfortunate as to separate themselves from the unity of the Body, or been excluded by legitimate authority for grave faults committed. “For in one spirit” says the Apostle, “were we all baptized into one Body, whether Jews or Gentiles, whether bond or free.” As therefore in the true Christian community there is only one Body, one Spirit, one Lord, and one Baptism, so there can be only one faith. And therefore, if a man refuse to hear the Church, let him be considered – so the Lord commands – as a heathen and a publican. It follows that those who are divided in faith or government cannot be living in the unity of such a Body, nor can they be living the life of its one Divine Spirit. (Mystici Corporis Christi, 22)

So this gives us the definition of the word ‘member’ as we were using it in the article. A member (in this sense) of the Catholic Church is a person who has been baptized and professes the true faith (i.e. and therefore is not a formal heretic), and has not separated himself from the unity of the Body (by entering a schism), and is not in the excommunicated state. Clearly then, a formal heretic is not a member of the Catholic Church, in that sense of the term ‘member’, because he does not profess the truth faith, and on account of his [formal] heresy has incurred latae sententiae excommunication, according to Can. 1364.

But does the formal heretic remain under the jurisdiction of the Church? Yes. Excommunication does not take the excommunicated person out of the jurisdiction of the Church. So in that sense, the formal heretic remains a Catholic, but not a Catholic in full communion with the Catholic Church, and thus not a member according to the necessary conditions listed in Mystici Corporis Christi 22.

I hope that helps answer your question.

from comment 724 here http://www.calledtocommunion.com/2010/06/how-john-calvin-made-me-a-catholic/comment-page-15/#comment-18536

From St. Isidore, bishop of Seville (570-636) who wrote a twenty-book topical encyclopedia titled Etymologies, we find that a good sign that a sect is not the Church Christ founded is that it is named after its founder, who is someone other than Jesus Christ. In Books VII and VII of hisEtymologies he says the following:

Haeresis is called in Greek from choice, because each one chooses that which seems to him to be the best, as in the case of the Peripatetic philosophers, the Academics, and the Epicureans and Stoics, or as others do, who, contemplating their perverse dogma, recede from the Church of their own will. And so heresy is named from the Greek from the meaning of choice, since each [heretic] decides by his own will whatever he wants to teach or believe. But it is not permitted to us [Catholics] to believe anything on the basis of our own will, nor to choose to believe what someone else has believed of his own will. We have the authority of the apostles, who did not choose anything out of their own will to believe, but faithfully transmitted to the nations the teaching they received from Christ. Even if an angel from heaven should teach otherwise, it would be called anathema. Sects are so called from following and holding [sequendo et tenendo]. Now sects are a habit of spirits, and are formed around a discipline or a proposal, holding to which they follow along, holding to different opinions from others in the cult of religion. Schism comes from the word for cutting. Schismatics believe in the same rite and the same cults as others, but they delight in separating from the congregation.

They are heretics who depart from the Church, calling themselves by the name of their author. … Arians are so called from Arius the presbyter of Alexandria, who did not recognize the Son as coeternal with the Father and asserted different substances in the Trinity, against which the Lord said “I and the Father are One.” … Priscillianists are so called after Priscillian, who in Spain composed a teaching which combined the errors of the Gnostics and the Manichees. … Pelagians are called after the monk Pelagius. These place free will ahead of divine grace, claiming that will is all that is needed to fulfill the divine commands. Nestorians are called after Nestorius, bishop of Constantinople, who claimed that the Virgin Mary was the mother, not of God, but of man, so that one person was made of the flesh, another of the divinity, and did not believe in One Christ in the word of God and in the flesh …. There are other heresies without founders and without names. Some of them believe that God has three forms, and others that the divinity of Christ can suffer. Others mark a point in time when Christ was born of the Father. Others do not believe that by the descent of Christ [into the netherworld] the freeing of all in the lower regions was accomplished. Others deny that the soul is the image of God. Others think that souls are changed into demons and animals of every kind. Others hold different opinions on the condition of the universe. Others thing that there are many worlds. Others think that water has existed as long as God has. Others walk about with unshod feet, while still others will share a meal with no one.

These heresies have risen against the Catholic faith and have been condemned by the apostles, the holy Fathers, or the councils. And while they are not in agreement with one another, being divided by many errors, it is with one name that they conspire against the Church of God. But whoever understands Scripture in any sense other than that which the Holy Spirit, by whom it was written, requires, even though he may not withdraw from the Church, may nevertheless be called a heretic.