The problem I have is this: how can the Church know that a person is in heaven? Public revelation ceased with the death of the last apostles, and the Church has never taught that the Pope receives a private revelation confirming that they are in heaven. If the Church can't know the person is in heaven, how can she infallibly declare it?
The answer is simple: The Spirit of God is Omniscient, He knows all things, and He informs His Bride of it, and She declares it on earth. God said, "whatever you bind on Earth will be bound in Heaven", and that means the Spirit of God will always guide such binding. Also, notice the way St. Alphonsus frames this: "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.” It is the Holy Spirit Himself who is infallible. The Roman Pontiff simply participates in the Holy Spirit's infallibility when canonizing Saints.
Miracles can serve as a divine testimony that the person is in heaven, but the Church is not infallible in judging if a miracle is legit or not. Therefore, the indirect way of knowing if the person is in heaven (by miracles) is itself not infallible.
Miracles are so certain a sign of divine testimony that they have converted unbelievers. Therefore, they most certainly can lead someone to certainty. Christ blamed the Pharisees for obstinacy mainly because they refused to convert even after seeing His miracles. Jehovah blamed Pharoah in the OT for the same reason; He saw miracles but refused to convert. But for faithful Catholics, it is not a miracle itself, however certain, that assures us of the supernatural, although a miracle like e.g. the Miracle of the Sun at Fatima, or a Eucharistic miracle, or a healing miracle worked by a canonized Saint or a Saint to be canonized, may convince unbelievers - it is the Church's authority to bind and loose that convinces Catholics. Christ assures us such a binding and loosing is bound by Him Himself in Heaven, so infallible.
In my opinion, that is a serious problem for the infallibility of canonizations and I have never heard a satisfactory answer to it.
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Ok. Read the Saints and Catechisms cited here. https://reasonstobechristian.com/f/are-canonizations-infallible-popes-saints-and-doctors-say-yes I hope they answer your question.
The
Baltimore Catechism confirms this, saying: "
#3. Q. 82. Why does the Church Canonize Saints?
A. The Church Canonizes Saints (1) to honor them, and (2) to make us certain that they are in heaven, and may, therefore, be invoked in our prayers.
Q. 83. Can the Church err in the Canonization of a Saint?
A. The Church cannot err in matters of faith or morals, and the Canonization of a Saint is a matter of faith and morals."
Things will be easier and simpler, both for Feeneyites, and for others, if we accept the Church is infallible. We are not. May God and His Holy Spirit guide us to all Truth. In Lord Jesus' Name. Mother Mary, pray for us sinners.