Are Canonizations Infallible?
If the Church declares someone a saint, how can we know with absolute certainty that he or she is in heaven?
While the Church has never officially defined canonization as an act of infallibility, the consensus of theologians has been that it is.
As St. Thomas Aquinas noted:
Since the honor we pay the saints is in a certain sense a profession of faith, i.e., a belief in the glory of the saints, we must piously believe that in this matter also the judgment of the Church is not liable to error (Quodlib. IX, a. 16).
Because the Pope is officially decreeing that all Christians are to believe that a certain person is in heaven, it is believed to fall under the definition of papal infallibility.