Is the Catholic declaration of an extra biblical saint infallible? Is the canonization of a person not found in the Bible (for example, Thomas Aquinas) an infallible decision? By this I mean that their sainthood can never be denied by any of the faithful without meaning a break with the Church's Magisterium.
The opinion on this issue seems to be divided.
According to the Catholic Encyclopedia, at least by the beginning of the XX century (i.e. pre Second Vatican Council) &quot;most of&quot; theologians thought canonisation involved an infallible claim. This is partly based on St. Thomas Aquinas's view:

&quot;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.&quot; (Quodlib. IX, a. 16)

However, there are divergent opinions. This much more recent article deals a great length with precisely this question. It starts by recognising the contemporaneity of aforementioned position:

Currently, it is the opinion of a majority of Catholic theologians that the canonization of Saints by the Pope is an exercise of papal infallibility. The Congregation for the Causes of the Saints (CCS) supports this opinion. And some Bishops also believe and teach this idea.

However, the author argues that actually, canonisation is not an infallible act. The author states:

Is the canonization of a Saint a doctrine of faith or morals? No, it is a judgment and decision, make by proper authority in the Church, that a person lived an exemplary holy life and was faithful to the teachings of Christ and His Church. Canonization is not a teaching, so it cannot fall under the teaching authority of the Church.

Why?

... the Magisterium is absolutely limited to teaching the truths found, explicitly or implicitly, in Tradition and Scripture. Therefore, the Magisterium is completely unable to teach that any person is a Saint (except for those persons mentioned in Tradition or Scripture). Neither the Pope himself, nor the entire Body of Bishops united with him, can teach that such a person is a Saint. The Pope cannot teach this infallibly, under papal infallibility, nor can he teach it even non-infallibly, under the Ordinary Magisterium. Likewise, the Bishops united with the Pope, even in an Ecumenical Council, cannot teach that such a person is a Saint. For the Magisterium is unable to teach truths found entirely outside of the Deposit of Faith.
Now the Saints who are mentioned in Tradition and Scripture, such as Saint Peter the Apostle, are a separate case. Since their lives and holiness is attested to in infallible Divine Revelation, the Church can infallibly teach their holiness and can infallibly declare them to be Saints. But most Saints have lived long after the canon of Scripture was closed. For unless the life of a Saint is a part of Sacred Tradition (e.g. the mother of the Virgin Mary), or unless a Saint is mentioned in Sacred Scripture (e.g. the father of the Virgin Mary, called Heli), such a Saint’s canonization cannot be considered a part of the teachings of the Church, nor of the Magisterium, neither infallibly nor non-infallibly.

More details on the article itself.