The Guadalupe account is a short and very carefully made piece of Nahuatl prose, and almost everything popularly known about Juan Diego comes from it.
The story
A man called Juan Diego, a Nahua from Cuauhtitlan, is crossing the hill of Tepeyac on his way to Mass in December 1531 when a young woman appears to him, speaking Nahuatl, and tells him to ask the bishop to build her a house on that spot. The bishop, Juan de Zumarraga, does not believe him. She sends him back. The bishop asks for a sign.
At the last encounter she tells him to gather flowers from the hilltop, out of season and out of place, and to carry them to the bishop in his cloak. When he opens the cloak the flowers fall and her image is on the cloth.
What kind of text it is
The account is the Nican Mopohua, printed in 1649 in a Nahuatl volume published by Luis Laso de la Vega, the chaplain at Tepeyac. It is in elegant classical Nahuatl and uses the rhetorical patterns of pre-conquest oratory. A year earlier Miguel Sanchez had published a Spanish account of the apparition.
The detail that carries it
The lady in the story speaks the language of the man she appears to, addresses him with the affectionate diminutives Nahuatl reserves for the humble, and chooses him rather than the bishop as her messenger. That is the point the text is making, and it is a large part of why the account travelled as far as it did.