What was the relationship between Mary and Saint Juan Diego?
In December 1531, the Virgin Mary appeared to Juan Diego on Tepeyac hill. She asked him for a church. Nican Mopohua; vatican.va, biography of Saint Juan Diego (2002)
What Tradition teaches
The Bible does not tell this. It is told in the Nican Mopohua, an account from the sixteenth century. The Church canonized Juan Diego in 2002 and celebrates Our Lady of Guadalupe on December 12 (Roman Martyrology).
What the Church teaches
Saint John Paul II said that the Virgin showed herself to Juan Diego as the Mother of the true God.
vatican.va, John Paul II, July 31, 2002 · Catechism of the Catholic Church
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Meet Mary and Saint Juan Diego.