The following is taken from "True or False Pope?", pp. 182-183
The 1917 Code of Canon Law provides that those who commit the following acts are suspect of heresy:
1. The propagators of heresy and those who participate with non-Catholics in divinis (can. 2316);
2. Those who contract marriage under the condition of having their offspring educated in a non-Catholic sect and those who have their children baptized by non-Catholic ministers or educated in a non-Catholic denomination (can. 2319);
3. Those who desecrate sacred hosts or species (can. 2320);
4. Those who appeal from the Pope to a general council (can. 2332);
5. Those who remain under sentence of excommunication for more than a year (can. 2340);
6. Those who administer or receive the Sacraments simoniacally (can. 2371).
In addition to these anti-Catholic activities specified under canon law, the highly respected commentary on the 1917 Code of Canon law by Wernz-Vidal also sets forth extra-canonical activities that are considered grounds for suspicion of heresy. They include taking part in the exercise of magic, charms or divination, and those who become members of sects which, whether openly or secretly, hatch plots against the Church.
In other words, a Catholic can propagate heretical doctrines, participate in false worship with non-Catholics, baptize, raise and educate their children in non-Catholic sects, commit sacrilege against the Blessed Sacrament, take part in satanic “black magic,” and formally join anti-Catholic sects and secret societies, and only be suspected of heresy. Even though these activities are objective mortal sins against the Faith, under the Church’s law they are only grounds for suspicion that one is a heretic.
Because the propagation of heresy is such a serious assault on the Faith of the Church, Fr. Augustine sets forth four categories to morally distinguish the types of “propagators of heresy” (can. 2316):
a) Credentes are such as externally profess the errors of heretics, e.g., by asserting that Luther or Döllinger were correct in their views, even though they may not know the particular errors of these leaders.
b) Receptores are those who receive and shelter heretics, especially with the intention of hiding them from the ecclesiastical authorities.
c) Fautores are such as favor heretics because of their heresy, by omitting to denounce them when required or demanded by their office, or by giving support to non-Catholic propaganda. This latter way of propagating heresy is followed by public and private persons who write for heretics, praise their methods and objects, recommend their work and give it material support, always provided that the heresy itself is the object of their mental and material favors.
d) Defensores means those who defend heretics for the sake of heresy, orally, in writing, or by acts of defense proper. All such persons are suspected of heresy if they act of their own accord and knowingly. Sponte is opposed to compulsion and fear, and therefore implies full deliberation and a free will not hindered by any extrinsic or intrinsic impediment, such as fear of losing an office, or one’s reputation, or customers. Scienter is opposed to ignorance, the object of which here is heresy, and means that these promotors or propagators of heresy must be aware that they are helping heresy as such.
Thus, one who “externally professes the error of heretics,” or who favors heretics “because of their heresy,” or who “defends heretics for the sake of heresy, orally, in writing, or by acts,” are only considered suspect of heresy. While a Catholic may be inclined to conclude that John Paul II, for example, was a heretic for worshiping with pagans and praising the errors of Martin Luther, the Church says these are only grounds for suspicion of heresy. One who commits these acts is not considered a “public heretic” (even if the scandalous acts are multiplied) and, if the person in question is a cleric, such activities do not deprive him of his ecclesiastical office.
 Augustine, A Commentary on the New Code of Canon Law, vol. VIII, bk. 5, (London: Herder Book Co., 1918), pp. 280.
 Augustine, A Commentary on the New Code of Canon Law, vol. VIII, bk. 5, pp. 288-289.