There is no domatic reason to believe it actually occured and it is the type of thing that would naturally be inserted by the author in their attempt to show how great Jesus was, very similar to the mostly fictional infancy narratives. It is certainly “true” but it is not necessarily the retelling of an actual event. Nowhere does Dei Verbum require us to believe in the literal historicity of the of every gospel story.
I am a little surprised the priest would bring up such a thing out of the blue as it can be unsettling to those not well versed in the church’s documents on scripture study.
Of course the Transfiguration occurred! This is a misapplication of the historical-critical method, which Pope Paul II warned about in 1964 in “The Historicity of the Gospels”
Some proponents of this method, motivated by rationalistic prejudices, refuse to recognize the existence of a supernatural order. They deny the intervention of a personal God in the world by means of Revelation in the strict sense, and reject the possibility or actual occurrence of miracles and prophecies. Some start out with an erroneous concept of faith, regarding faith as indifferent to, or even incompatible with, historical truth. Some deny, a priori as it were, the historical nature and historical value of the documents of Revelation. And finally, some minimize the authority of the Apostles as witnesses to Christ. Belittling their office and their influence in the primitive community, these people exaggerate the creative power of the community itself.
All these opinions are not only contrary to Catholic doctrine, but also devoid of scholarly foundation and inconsistent with the sound principles of the historical method.
They deny that miracles occur, or that Jesus performed miracles. They either make up alternative explanations for the miracles of Jesus (the feeding of the 5000 was about sharing, people there had hidden food and Jesus convinced them to share, etc.), or if they can’t think up an alternative explanation, they just claim that it never happened, it was inserted as a literary device (Transfiguration, infancy narratives, etc.)
They seem to think that God can’t act outside of their preconceived, rationalistic laws of science - as if the God that created the Universe out of nothing would have trouble causing a virgin to become pregnant (no, they say, that’s scientifically impossible).
Even the people that believe that the miracles in the Bible are fiction, midrash, or whatever they call it - amongst themselves they always caution each other not to talk that way to general audiences, because they’ll be misunderstood - so I’m surprised to hear that it was preached in a homily.