The most common religion in the United States is Christianity which comprises of Protestants and Catholics. They make up at least 60% to 70% of the population with other religions like Islam, Judaism, Hinduism and Buddhism following behind. Check out http://en.wikipedia.org/wiki/Religion_in_the_United_States, for more details.
Christians are the most common, with Catholics being the most numerous Christians. There are actually more Protestants than Catholics but Protestants are fragmented into smaller groups (baptists, presbetyrians etc).