Hmm, after further thought, maybe I am just guilty of the sin of pride on this issue. Nevermind.
This was a very brave thing to say, Frank.

I would, though, like to address the concerns you mentioned, just to help lay them further to rest.
I don’t disagree with anything you’ve said, although, through NO fault of your own, you quoted my post before I edited it. I guess, while I have no trouble whatsoever in accepting the Church’s authority to pass down divinely revealed truth that has its origins with Jesus and the Apostles, and to speak infallibly when talking ex-cathedra in matters of faith and morals, is it possible, according to CATHOLIC teaching, that the Church may be wrong on accepting Our Lady of Lourdes as a valid apparition? I mean, after all, do I need to mention the name Galileo(sic?)
This is not the same thing, though, Frank. Galileo’s case was a scientific issue. He was condemned because he said that, scientifically speaking, the cosmology of the Bible is wrong, and this placed him in conflict with the 17th Century Catholic Church, which was, at the time, dealing with the Protestant revolt and on guard against major outbreaks of heresy. The fact that Galileo’s scientific opinion was correct has no bearing on the Church or its authority, for, we do not believe that the Church can speak infallibly in terms of science, but only in terms of theological truth. Theologically, Galileo was not correct, and the Church was.
As for a private revelation like Lourdes, etc., the Church can rule with certainty about such things because of its theological authority; and, please recall that all the Church says about Lourdes and other approved private revelations is that they are “worthy of belief.” What this means is that the Church has investigated the claims of this supposed private revelation and has checked them up against that which has been “pass down as Divinely revealed from Jesus and the Apostles,” and has ruled that there is nothing in this private revelation that contradicts or is not in accord with the content of the Apostolic Deposit. Thus, when the Catholic Church approves of something like Lourdes, it is acting within the limits of its Christ-given authority and making a judgment about the nature of the Apostolic Deposit, and that which conforms to the Apostolic Deposit. This is within the Church’s full authority to do because the Catholic Church is the custodian and final arbiter of what does and does not conform to Apostolic teaching.
The Church has formally declared that Lourdes conforms to Apostolic teaching, and it is thus declared to be “worthy of belief.” While a Catholic is not required to believe in Lourdes (or any private revelation), the Church, using its Christ-given authority to discern sound Apostolic doctrine, has declared that there is no obstacle in place if a Catholic wishes to believe in it; nor is a Catholic who chooses to reject Lourdes able to fault his brother Catholic for believing in it. Indeed, you are perfectly free to reject Lourdes if you wish to; but just don’t seek to deny the freedom of others to believe in it, for that would be denying that the Catholic Church has the authority to decide what does and does not conform to the Apostolic Deposit. Either the Catholic Church is the final custodian and arbiter of the Apostolic Deposit or it is not. If it is, then we have no right to question its decision to declare Lourdes to be in substantial accord with the Apostolic Deposit.
While we are to trust in the Lord and lean not on our own understanding(Proverbs 3:5) is there any Catholic teaching that infallibly guarantees the Church’s accuracy in validating post-apostolic apparitions?
Yes. See Matt. 16:19, John 14:16-17 & 16:13, and 1 Thess. 5:21.
The Hail Mary says, “The Lord is with you.” It says, “Blessed is the fruit of thy womb Jesus.” It says, “Mother of God.” This “Marian” apparition, however, according to what you are saying, PERSONALLY made no such statements, which brings us back to square 1 on the 1 John 4:1-3 litmus test.
Notice that the Greek verb “test” (both in 1 John 4:1 and 1 Thess. 5:21) is in the “you-PLURAL.” This is because the Apostles are referring, not to the personal discernment of an individual, but to the collective authority of the Church itself. The Catholic Church has “tested” Lourdes, and has judged that it should be approved. It has also tested countless other supposed apparitions of the Virgin, and judged that these should not be approved. Thus, 1 John 4:1-3 and the Catholic Church are in perfect accord. As for the Virgin’s statement at Lourdes (“I am the Immaculate Conception”), as had already been posted here this statement was made a couple years after the Immaculate Conception of Mary was formally proclaimed to be a dogma of the Catholic Church.
continued. . .