Because it became (perhaps by divine design) the single, state religion of one of the greatest and long lived early empires - the Roman Empire.
Christianity.
Christianity!
Christianity.
In Phillpines
Christianity.
Christianity.
christianity
Christianity
Hinduism is the major religion of India. If your question is about the U.S. state of Indiana, the major religion is Christianity.
Christianity spread through migrant diffusion when European countries brought it when they settled in America.
Catholicism is the largest denomination within Christianity. Christianity itself is the largest religion in the entire world, hence Catholicism is a major world religion.
No, Islam is the major religion in Iran.