Mithrandylan:
That the pope is infallible when defining for the whole Church a matter of faith and morals with his authority is a dogma of the faith. Are you familiar with Vatican I?
Do canonisations count as Faith and Morals? Can the sainthood of this or that person be said to be "Catholic teaching"? Does one fall into heresy for not regarding this or that person as a saint...?
Canonizations would at least fall under morals if not faith. We are bound to honor the saints, and to refuse to honor a saint would be a sin against charity at least. Saints are also incorporated into the liturgical life of the universal Church, and the Church cannot incorporate something unholy into the liturgy. Combined with the language used in these canonizations, I think they fall under the heading of papal infallibility, yes.
But even if they didn't fall strictly under
papal infallibility, canonizations are regarded as infallible by theologians under the Church's infallibility:
The Church's infallibility extends to the canonization of saints. This is the common opinion today.
Canonization (formal) is the final and definitive decree by which the sovereign pontiff declares that someone has been admitted to heaven and is to be venerated by everyone, at least in the sense that all the faithful are held to consider the person a saint worthy of public veneration. It differs from beatification, which is a provisional rather than a definitive decree, by which veneration is only permitted, or at least is not universally prescribed. Infallibility is claimed for canonization only; (20) a decree of beatification, which in the eyes of the Church is not definitive but may still be rescinded, is to be considered morally certain indeed, but not infallible. Still, there are some theologians who take a different view of the matter.
Proof:
1. From the solid conviction of the Church. When the popes canonize, they use terminology which makes it quite evident that they consider decrees of canonization infallible. Here is, in sum, the formula they use: “By the authority of our Lord Jesus Christ and of the apostles Peter and Paul and by our own authority, we declare that N. has been admitted to heaven, and we decree and define that he is to be venerated in public and in private as a saint.”
2. From the purpose of infallibility. The Church is infallible so that it may be a trustworthy teacher of the Christian religion and of the Christian way of life. But it would not be such if it could err in the canonization of saints. Would not religion be sullied if a person in hell were, by a definitive decree, offered to everyone as an object of religious veneration? Would not the moral law be at least weakened to some extent, if a protégé of the devil could be irrevocably set up as a model of virtue for all to imitate and for all to invoke? (117-18, emphases added)
Since the honour we pay the saints is in a certain sense a profession of faith, i.e., a belief in the glory of the Saints [quâ sanctorum gloriam credimus] we must piously believe that in this matter also the judgment of the Church is not liable to error.
As to your question if denying this makes you a heretic, it depends on who you ask.
To suppose that the Church can err in canonizing, is a sin, or is heresy, according to St. Bonaventure, Bellarmine, and others; or at least next door to heresy, according to Suarez, Azorius, Gotti, etc.; because the Sovereign Pontiff, according to St. Thomas, is guided by the infallible influence of the Holy Ghost in an especial way when canonizing saints.”
If anyone dared to assert that the Pontiff had erred in this or that canonisation, we shall say that he is, if not a heretic, at least temerarious, a giver of scandal to the whole Church, an insulter of the saints, a favourer of those heretics who deny the Church’s authority in canonizing saints, savouring of heresy by giving unbelievers an occasion to mock the faithful, the assertor of an erroneous opinion and liable to very grave penalties.
Such a denial would accompanied by mortal sin according to Pope Benedict and others. Even if it is not necessarily heresy (and they do not rule out the possibility that it is) it still gets you to the hot place.