The South is Becoming a New Catholic Stronghold

[Sol]

Fastest growth by percentage doesn't equal "becoming a new catholic stronghold." In the typical rural Southern county, if 20 Mexicans move in (which is not such an uncommon thing these days) the Catholic population may well double. They're still a negligible population

If 20 Hispanics move in and the Catholic population doubled, it very well could be that there were only 10 or even 5 Catholics there before.  Especially in the areas where immigrants are coming from, Catholicism in Latin America is being replaced by Protestantism, especially of the Pentecostal variety.  And that's not to mention the quaintness of equating Hispanics with Mexicans.  While they are still by far the largest group, their share of illegal immigration has been going down.

1. You're making the same mathematical mistake, but in reverse. Protestant denominations are growing quickly by percents, but Latin America remains overwhelmingly Catholic. The 2010 Mexican Census found that 83% of the population was Catholic.
http://en.wikipedia.org/wiki/Mexico#Religion
2. Why do you presume Hispanics, Mexicans or otherwise, are illegal? That's quite an onerous suggestion.

Some central american countries are at around 40% protestant, though.