It’s seen as a sin. What level: Venial or Mortal?
CIC Can. 1364 §1. Without prejudice to the prescript of can. 194, §1, n. 2, an apostate from the faith, a heretic, or a schismatic incurs a latae sententiae excommunication; in addition, a cleric can be punished with the penalties mentioned in ⇒ can. 1336, §1, nn. 1, 2, and 3
CIC Can. 751 Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him.
Jimmy Akin writes:
“a person is not subject to the penalty if they committed their offense through innocent ignorance, inadvertence, or error (1323 no. 2).” and “If a schismatic lacked sufficient knowledge of the moral character of what they were doing (e.g., they didn’t realize that being a Catholic was important or they didn’t realize that what they were doing was actually schimatic) or if they didn’t give deliberate consent to the act (e.g., because they were suffering from a severe psychological illness that prevented them from deliberately consenting to any of their actions) then they would not be in mortal sin. They would still have sinned gravely, but the sin would not be mortal.”
jimmyakin.com/2006/09/schism_and_mort.html
Can. 1323 The following are not subject to a penalty when they have violated a law or precept:
1/ a person who has not yet completed the sixteenth year of age;
2/ a person who without negligence was ignorant that he or she violated a law or precept; inadvertence and error are equivalent to ignorance;
3/ a person who acted due to physical force or a chance occurrence which the person could not foresee or, if foreseen, avoid;
4/ a person who acted coerced by grave fear, even if only relatively grave, or due to necessity or grave inconvenience unless the act is intrinsically evil or tends to the harm of souls;
5/ a person who acted with due moderation against an unjust aggressor for the sake of legitimate self defense or defense of another;
6/ a person who lacked the use of reason, without prejudice to the prescripts of cann. ⇒ 1324, §1, n. 2 and ⇒ 1325;
7/ a person who without negligence thought that one of the circumstances mentioned in nn. 4 or 5 was present.