Luis de Mena, casta painting, c. 1750. Museo de América, Madrid.
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He's speaking - in part - about that which is depicted in the painting shown above.
The Blessed Virgin appeared as a mestiza to an indigenous man, now venerated as St. Juan Diego.
For more on casta paintings go here.
Ed. note: When I was little, my grandmother visited Guadalupe and Mexico quite often. She lived in San Francisco and often visited the California Missions as well. She sent books and artifacts and devotional items from each of these places - just for me, which probably accounts for my love of Spanish Colonial and Santero art, including a great love for Latino and indigenous people... and now our beloved Pope. :)