Question from a Reader:
An argument widely used by sedevacantists to prove their thesis is the infallibility of canonizations (so, due to the dubious recent canonizations, it would prove that such Popes are antipopes) they quote the letter "Inclita pictavorum" by Pius XI to prove this infalibility:
“(…) Although we have expressed our desire to grant this Blessed One (André-Hubert Fournet) the sovereign honors of the triumphant Church, […] however, as the matter is very serious, because it is linked to the INFALLIBLE MAGISTERIUM of the Vicar of Christ , We believe it opportune to postpone this canonization, in order to ask, according to the custom of the Roman Church, in a semi-public Consistory, for your second opinion, from Our venerable Cardinal Brothers, as well as from all those who should participate in this Consistory, patriarchs, archbishops, bishops and abbots nullius.
[...] We, after having pleaded again and with more fervor the lights from above, have, as Supreme Head of the Catholic Church, pronounced the INFALLIBLE sentence in these terms:
“In honor of the Most Holy and indivisible Trinity, for the exaltation of the Catholic faith and for the growth of the Christian religion, for the authority of Our Lord Jesus Christ, of the blessed apostles Peter and Paul, and for ours; after mature deliberation, and having often begged for divine help, in the opinion of Our Venerable Brothers the cardinals of the Holy Roman Church, the patriarchs, archbishops and bishops present in the city, we have decreed and defined Blessed André-Hubert Fournet and included in the catalog Santos; deciding that his memory be celebrated every year with godly devotion in the universal Church on the day of his birth in heaven, that is, on May 13, among the holy confessors and non-pontiffs. In the name of the Father and the Son and the Holy Spirit. ""
Can you answer this Pius XI document? (or do you already have an answer?)
There are two questions implicit in this Sedevacantist argument. 1) Are canonizations infallible? 2) If they are, do the recent controversial canonizations prove that the Popes who canonized these individuals violated infallibility, and hence were not true Popes?
The short answer to the first question is that the Church has never defined that infallibility extends to canonizations, which are a secondary object of infallibility. The First Vatican Council only defined that Papal infallibility extends to the primary object of infallibility (Mansi, 52,1226f), which are truths that have been formally revealed by God, and which are contained in Scripture or Tradition. The Council intended to address the secondary objects of infallibility, and even drafted a document for discussion (Mansi. 53,313.316), but was cut short by the Franco Prussian war and never completed it.
Because of that, it is not infallibly certain that canonizations are infallible, but it is the common opinion of theologians that canonizations are infallible. In the letter you quoted, Pius XI was simply stating what is commonly believed. He was not infallibly defining that canonizations are infallible.
But assuming they are in fact infallible, it would not prove the recent Popes were not true Popes. What it would prove is that the controversial individuals they canonized died in the state of grace and arrived at the beatific vision prior to their canonization. The object of the infallible judgment is that the individual is in heaven, not that they lived a life of heroic virtue. Many people are converted at the hour of death, die in the state of grace, and end in heaven - such as the Good Thief, Saint Dismas.
Considering that John XXIII, Paul VI, and John Paul II all died with the sacraments, and with millions of Catholics praying for them, it seems entirely likely that they would have died in the state of grace. And considering the number of prayers and masses offered for them after they died, it is certainly possible for them to have arrived in heaven before they were canonized. Therefore, the fact that they were canonized does not prove that the Pope who canonized them is not a legitimate Pope.
1) The Church has never definitively declared that infallibility extends to canonizations, which are a secondary object of infallibility.
2) If the common opinion is correct, and canonizations are infallible, the Sedevacantists would still have to prove that the person canonized was not in Heaven, in order to prove that the Popes who canonized them violated infallibility.
 Also see the official relatio delivered by Bishop Gasser, spokesman for the deputation de fide, during Vatican I. Published it: O’Connor, The Gift of Infallibility, Ignatius Press, 2008, p. 80).