I will limit myself to three observations. First, although it seems shocking to deny the existence of someone so well known, it is an easy thing to do. Take a comparison from U.S. history. Suppose I assert that Davie Crockett never existed, that he was just a legend people made up. You might think I'd gone crazy, but then how would you prove me wrong? You could show me any number of books, drawings, documents about Davie Crockett and I could dismiss them by saying they came about as a result of a conspiracy between the Tennessee Chamber of Commerce and a certain hat manufacturing company. It would take a trained historian to evaluate the evidence and give a solid opinion.
Second, experts have been investigating the life of Juan Diego. It is called the canonization process. The fact that in 1990 it got to the point of beatification (declaring him Blessed Juan Diego) indicates a careful review of evidence.** The opinion of the cleric I referred to above was expressed to the Congregation for Saints and, because of his association with the Basilica, it surely has been seriously considered.
Third, there are many books which explain the historical context of the apparitions to Juan Diego. A good reference is Our Lady of Guadalupe by Dr. Warren H. Carroll who has a Ph.D. in history from Columbia. Altho he wrote the book before the current controversy, it does present some convincing evidence. For example he cites the will of Juana Martin of Cuauhtitlan, a relative of Juan Diego. This nearly contemporary document dated March 11, 1559, mentions Juan Diego and declares:
As I mention in a homily on Our Lady of Guadalupe the apparition occured at a time many, including priests, were questioning the use of images. That, along with the conditions in the early years of the colony in Mexico, would explain some of the reticience in historical documents. Nevertheless there is sufficient evidence that we do not have to doubt the historical existence of Blessed Juan Diego.
*I say this not as a matter of faith. It is possible to be a good Catholic and not accept the apparitions of Guadalupe - or Lourdes or Fatima or any other. Rather as a Catholic open to the possibility of a marian apparition, I consider there is excellent evidence that the events of Tepeyac did actually happen. The foremost evidence is the tilma itself. In a world with many scientific-religious puzzles, it is one of the most fascinating.
**The Houston Catholic Worker summarizes some of the evidence. They quote Fr. Virgilio Elizondo, who has made a life-long study of Our Lady of Guadalupe:
"some historians who question the authenticity of the entire Guadalupe tradition think that he is simply a legendary character created by the accounts of the apparitions first published in 1649 by Luis Laso de la Vega. Nothing could be further from the truth. First of all, the accounts published in 1649 were like an edited book composed of accounts that had been written much earlier. The earliest account of the apparitions and of Juan Diego's role in them was written by the native Nahuatl scholar Antonio Valeriano in the late 1500's-only a few years after the apparitions. This account is known as the Nican Mopohua. It is more probable that he recorded in a very precise and poetic way the account narrated by Juan Diego himself. Another account, the Nican Moctepana, was written by Mestizo Fernando Ixtlilxochitl in 1590. These manuscripts of the earliest account, along with many others, can be found in the Lennox collection of the New York Public Library."
Juan Diego: Some Historical Evidence
Homily about Blessed Juan Diego