Spanish missionaries had been trying to spread Christianity in the Americas since the end of the 15th Century with very limited results.
In the 16th Century, masses of Christians were leaving the Catholic Faith due to the Protestant Reformation.
In 1531, not long after after the beginning of the Protestant Reformation, something happened in Guadalupe, a location which could be seen as the center of the American continents. This vision resulted in 8 million conversions to Catholicism within a seven year time span! Catholicism has continued to be the main religion in Central and South America, and is generally practiced with great fervor and devotion.
So we can ask ourselves, what exactly happened in Guadalupe? If we believe what the Church and the people say, it was a vision of Mary. Where there is a vision, there is also a visionary. The Church and the people say that it was St. Juan Diego. If we dismiss this claim due to lack of concrete historical evidence of the man, we still have to ask ourselves who the visionary was.
If we say that nothing miraculous happened in Guadalupe, then we are challenged to explain the phenomenon of such incredible amounts of conversions over such a brief time-span. Was it a scam created by the Spanish? Obviously not, because their track record in the Americas certainly makes such a notion lack credibility. Plus, if this was something concocted by the Spanish then they would have created documents to support their claim. In other words, the lack of concrete historical evidence to support the existence of St. Juan Diego works both ways. Objectively speaking, it can also argue against his existence as a real person, but it can argue against his existence as a fabrication.