Although we should not try to do only the minimum necessary to remain Catholic, there is a line between Catholic and falling away from the Faith and leaving the Church.
Let us consider our duty as laid out in Canon 1325 of the 1917 Code of Canon Law: “The faithful are bound to profess their faith publicly, whenever silence, subterfuge, or their manner of acting would otherwise entail an implicit denial of their faith, a contempt of religion, an insult to God, or scandal to their neighbor.” This Canon describes the three sins that remove one from the Catholic Church as we saw in the section on prophesy considering the abomination of desolation.
Notice that we can commit heresy, apostasy or schism in several ways: silence, subterfuge or manner of acting. Many think of heresy, such as some of the 95 Theses Martin Luther posted on the door of the church at Wittenberg. However, a refusal to genuflect in a church before the Blessed Sacrament could signal a denial of the doctrine of the Real Presence by our manner of acting.

All apostates from the Christian faith and each and every heretic or schismatic incur the following penalties:
1. ipso facto excommunication;
2. If they have been admonished and to not repent, they shall be deprived of any benefice, dignity, pension, office or other position which they may hold in the church; they shall be declared infamous, and, if they are clerics, they shall after renewed admonition be deposed;
If they have joined a non-Catholic sect or have publicly adhered to it, they incur infamy ipso facto, and, if they are clerics and the admonition to repent has been fruitless, they shall be degraded. Canon 188, paragraph 4, provides, moreover, that the cleric who publicly abandons the Catholic Faith loses every ecclesiastical office ipso facto and without any declaration, Canon 2314, paragraph 1

From the above Canon we can see that joining a non-Catholic sect is an heretical manner of acting. And so this is obviously a sin that removes one from the Catholic Church.
It is not a sin of heresy to ask a reasonable question about a point one does not in matters of Faith. One must have the attitude of accepting all that God has revealed and teaches through His holy Church.
The question arises, can one appear to be a heretic or schismatic without willfully being one? And the answer is yes. We can give the appearance of heresy and/or schism by our manner of acting or even our speech, by innocently saying something against the Faith or obedience due to the Church.
How will the Church proceed in such a matter? Normally our pastor or Bishop would warn of us our error so that we could amend. If we refused to amend, a second warning would proceed. If we still refuse, we are obviously not innocent, but guilty and the Bishop can declare us excommunicated. And notice the words declare us. By the act of apostasy, heresy or schism, we leave the Church, even without warning.
Now a single or apparently innocent act should not be judged as heresy or apostasy, but notorious consistent actions should be. Canon 2200 provides: “The evil will, spoken of in Canon 2199, means a deliberate will to violate a law, and presupposes on the part of the mind a knowledge of the law and on the part of the will freedom of action. Given the external violation of a law, the evil will is presumed in the external forum until the contrary is proved.” Basically if someone looks like a heretic, then under the law we presume that he is a heretic.
The Bull Cum Ex Apostolatus Officio of Pope Paul IV in paragraph 6 states: “We add moreover that if at any time it shall appear that any Bishop even if he assert for himself the rank of Archbishop, Patriarch, Primate, or Cardinal of the aforesaid Roman Church, or legate, or even Roman Pontiff before his promotion or assumption into the Cardinalate or Pontificate, shall have deviated from the Catholic faith or have fallen into any heresy, or incurred, excited or committed any schism, his promotion or assumption even made in full concord and with the unanimous consent of all the Cardinals, shall be null, abrogated, and void, nor shall be called or become valid even by the reception of the grace of consecration nor the subsequent possession of government and administration, nor even by the enthronization or adoration of the elected person as Roman Pontiff or the universal obedience rendered to him for how long a period soever.”
Notice the word appear. The reason the Church judges on outward appearances is that we cannot know what is in a person’s heart. For her own protection, the Church must judge the outward appearances, when they show a consistent pattern of heresy or schism.
Let us take a case where our pastor stands up in the pulpit and declares that Jesus is not present in the Blessed Sacrament. After the sermon, we question him on this and he holds to his heretical position. Saint John Chrysostom tells us: “Wicked? In what sense? If indeed in regard to Faith, flee and avoid him; not only if he be a man, but even if he be an angel come down from Heaven; but if in regard to life, be not over-curious.” We are to flee and avoid him. In fact, we have a duty to inform his superior, our Bishop of this problem, so that the Bishop can either correct him or remove him.
And what if, God forbid, the Bishop does nothing? Then we have the right and duty to go over his head to the Pope and ask him to take action. In the mean time we will flee these men, because instead of being shepherds, they have become wolves in sheep’s clothing and a danger to our souls.