Like the previous answer stated, Christianity is the dominant religion in Peru. It should not be surprising that Catholocism is the main religion. Spain colonized Peru and landed there in 1531. Spain was the strongest Catholic country in Europe during the reformation. What is surprising, however, is the rapid rise of Protestants not only in Peru, but also South America. The sociologist Peter Berger documents this very clearly. Also with the rise of protestantism, there is also a burgeoning middle class. With that said, we should also acknowledge rich religious tradition of Peru with the Incas.
The Constitution of Peru allows for religious freedom, so Peru has no official religion.
Peru was conquered by Spain in the 1500s and was a colony of Spain until the early 1800s. For this reason, its population, along with those of all other former Spanish colonies, is heavily Roman Catholic.
According to the CIA Factbook, over 81% of Peruvians are Roman Catholic. The Factbook also lists another 12% or so as evangelical Christians. Evangelical protestant sects are gaining ground throughout Latin America these days. There are populations of Seventh-Day Adventists and Mormons, but the Peruvian national census only includes categories for Catholic, evangelical, other, and none so there are no official numbers.
Finally, there are still some Peruvians, mostly indigenous, who practice indigenous pagan religions.
Travelling through Peru, one thing that I noticed was that the various Protestant sects are much more visible in poorer areas and in new areas of towns, particularly in the Amazon basin and in cities like Iquitos. Here, there often wouldn't be a Catholic church at all, but you would see competing churches from Jehovah's Witnesses, 7th Day Adventists, and others. I presume this is because the bureaucracy of the Catholic church makes it "slower moving" to open new churches.