Let's think about what actually happened, shall we? Recall that when Our Lady of Guadalupe first appeared to Juan Diego, the latter was on his way to Mass I emphasized that last part to point out that Juan Diego was already Catholic when Our Lady approached him. She instructed him to go to the bishop to have a cathedral built at that site. Again, let's consider that there was already a diocese, with bishop, established at that locale and most likely, nearby areas had their own diocesan structures.
So quite a bit of Catholic groundwork was already laid by the time of Our Lady's appearance. Who laid that groundwork? Why, it was those big bad conquistadors! They played a large, and overlooked, part in ending human sacrifice in Mexico. They paved the way for missionaries to come and proclaim the Faith to natives such as Juan Diego and his family. They paved the way for the miracle of Our Lady of Guadalupe.