[onedoug]

Because he is attempting to change what has been Church doctrine since its inception. And if it hasn’t been, what has been the point of the Church in general in this matter for the past 2000+ years?

[scouter]

Because he is attempting to change what has been Church doctrine since its inception.

Attempting to change doctrine and succeeding at changing doctrine are two different things. No one said the Holy Spirit would prevent popes from attempting to change doctrine, only that they wouldn't succeed.

A judicial process is not doctrine, however much it may undermine doctrine. We are not guaranteed that the pope will be smart, competent, effective, a good preacher, that he will smile a lot, or even be a good person. We are only guaranteed that he will not teach as true a matter of faith or morals that is not true.

Papal infallibility is actually quite narrow, and quite rare. For a pope to be infallible he must:

1. Be speaking about a matter of faith or morals;
2. Be speaking to all Christians of all times;
3. Intend to be speaking infallibly and make that intention crystal clear.

The last time that happened was when Pope St. John Paul II said that the Church does not have the authority to ordain women priests. Before that was in 1950 when when Pope Pius XII defined the doctrine of the Assumption of the Blessed Virgin Mary.

Here's the language Pope Pius XII used to invoke the charism of infallibility:

For which reason... by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that ....

Here's how John Paul II put it:

Wherefore, in order that all doubt may be removed regarding a matter of great importance, a matter which pertains to the Church's divine constitution itself, in virtue of my ministry of confirming the brethren (cf. Lk 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful.

In both cases they made it clear that it was a matter of faith and/or morals, that they intended to invoke infallibility, and that all Christians now in the future were bound to hold that belief.

There is still debate among orthodox—that is to say, "right believing"—theologians about whether or not canonizations are infallible.