"The meaning couldn't possibly be more clear:"
Yes, and you couldn't possibly be more wrong about it.
To say that a particular group of people are so obdurate that not even an apparition would turn them around is not the same thing as saying that there will never be apparitions.
As a matter of fact, unless you wish to deny the Transfiguration, we know that there have been apparitions.
The transfiguration isn't the same and belongs in a unique category as a one-time thing. Even after his resurrection, Jesus seems to have appeared in normal human form except for his wounds. I don't know exactly what Paul saw in his vision other than blinding light.
The scriptures do talk about spirits and testing them. The test as described in the scriptures has never been applied to any of the apparitions I've studied. The spirit is supposed to affirm that Jesus is the Christ the son of the Living God. There could be other spirits named Jesus.
You mean if Petronius denies the Transfiguration, you won't know that there have been apparitions? No comment...:)
Jesus was there during the Transfiguration, and He has never been reported as being present during a Marian event. Only apparitions calling themselves 'Mary' have been involved in these stories, and her 'son's' continual absence is rather noticeable.