South america is dominated by what subgroup of christianity

The answer is: Roman catholics

In the past, both the king of the Spanish empire and the leader of its armada was a devout Roman catholics.

When the spanish empire conquered south america and made it into their colony, they mandated the people of south america to use spanish as their main language and make roman catholics as their national religion. Which is why south america is dominated by that subgroup of christianity.