During one of Pope Francis' speeches on his landmark trip to Mexico earlier this year, he issued a cryptic admonishment. In an address to Mexican bishops, the pontiff said that he was "particularly concerned about those many persons who, seduced by the empty power of the world, praise illusions and embrace their macabre symbols to commemorate death in exchange for money."

For those outside of Latin America, the pope's reference to a secretive "cult" that venerates Santa Muerte, or "Saint Death," is likely to escape notice. But to the 10 million to 12 million adherents in the region, the pope's criticism of Santa Muerte, which has challenged the influence of the Catholic Church, was clear.

"As esoteric as [devotion to Santa Muerte] seems, it is very practically oriented," Andrew Chesnut, the Bishop Walter F. Sullivan Chair in Catholic Studies at Virginia Commonwealth University and author of "Devoted to Death: Santa Muerte, the Skeleton Saint," told The National Catholic Review.

Santa Muerte, and the people who worship her, are much maligned, often associated with the region's narco underworld. But the reality, like faith, is more complex.