The following is taken from True or False Pope? pp. 183-185
In his commentary on the 1917 Code of Canon Law, Fr. Augustine explains the canonical process and penalties for those who are suspect of heresy:
We now proceed to the penalties the Code inflicts on those suspected of heresy.
a) They must, first, be warned, according to canon 2307, to remove the cause of suspicion. A reasonable time should be granted for this purpose in the canonical warning.
b) If the warning proves fruitless, the suspected person must be forbidden to perform any ecclesiastical legal acts, according to can. 2256. If he is a cleric, he must be suspended a divinis after a second warning has been left unheeded.
c) If, after the lapse of six months, to be reckoned from the moment the penalty has been contracted, the person suspected of heresy has not amended, he must be regarded as a heretic, amenable to the penalties set forth in canon 2314. Whilst the penalties enumerated under (b) are ferendae sententiae, to be inflicted according to can. 2223, 3, the penalties stated under (c) are a iure and latae sententiae.
Note that, since the ferendae sententiae penalties require a canonical warning and a clear statement of the time granted, the moment from which the penalty is contracted can be almost mathematically determined.
As we can see, under the Church’s positive law, if one commits an act that renders him suspect of heresy (e.g., externally professing heretical errors, worshiping with pagans, profaning the Eucharist, etc.), he is not considered a heretic until the above procedures have been followed. In his commentary on the 1917 Code, Fr. Henry Ayrinhac notes that a person’s actions may render him suspect of heresy de facto, yet “the suspicion has no canonical effect until the warning has been given.” He then goes on to explain the procedures and penalties:
If a person who is suspected of heresy does not, after being duly warned, remove the cause of the suspicion, supposing that it is morally possible to do so, he should be debarred from the legitimate acts. A cleric should receive a second warning, and if this too remained fruitless he should be suspended a divinis. After inflicting these punishments, six months more may be allowed, and if at the end of this time the party suspected of heresy has shown no signs of amendment, he is to be considered as a heretic and punished accordingly.
We can see the prudence and patience of Holy Mother Church with regard to such people. Under the Church’s law, a prelate is not considered a heretic for engaging in any of the aforementioned activities until he has been duly warned by legitimate authority, and may not even be suspended from his official duties until he fails to heed a second warning. Further, the prelate must be given six months more to remove the cause for suspicion, and after that he is only considered a heretic by the Church if he shows no signs of amendment. Finally, even after the six month period in which amendment is to take place, if the suspect is a cleric, he is not immediately suspended or forbidden from performing ecclesiastical legal acts. Such penalties are not automatic, but must be imposed, ferendae sententiae, by the proper ecclesiastical authorities, who may have reason to refrain from putting them into effect.