They were called Missions, and were built by Franciscan or Jesuit missionaries.
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You mean the buildings? That would be the Cathedral of the city of Puebla, which is also the largest cathedral in Latin America, with a bell tower over 70 meters (230 feet) high.
If you mean the actual faith, that would be Roman Catholicism, practiced by 88% of the population.
There are many as the most prominent religion is Roman Catholicism with chapels, churches and cathedrals in most population centers.
Literally, thousands of them. Even the smallest village has a small chapel to pray.