The largest religions in the United States are Christianity (including both Protestantism and Roman Catholicism) and Judaism. Among Americans professing a religious faith, about 47 percent are Protestant, 21 percent are Roman Catholic, nearly 2 percent are Jewish, 1 percent are Muslim, and 26 percent practice other religions, including Buddhism, Hinduism, and hundreds of others. Only the major religious groups (in terms of numbers of adherents) are discussed here.

Hinduism is the oldest of the world's organized religions and the only one without a founder. It began in India sometime during the second millennium B.C., when its sacred texts, called the Vedas, were written and collected. Hinduism has one supreme God but recognizes many other gods and goddesses. It believes in reincarnation and the law of karma, or cause and effect. It has about 200 sects.

Buddhism developed in India in the sixth century B.C. as a reform movement within Hinduism. Its founder was Siddhartha Gautama, known as Buddha. The religion spread into other Eastern countries and largely disappeared from India. It also believes in reincarnation and the law of karma. Buddhism has two main branches—Theravada Buddhism and Mahayana Buddhism—and numerous sects with varying practices and beliefs.

Judaism had its beginnings among nomadic Jews about the 14th century B.C., but did not reach developed form until many centuries later in Palestine. It is a monotheistic religion based on the idea that the Jews are chosen by God to be a priest-people to the world. The Torah is the commandment of God to the Jewish people and also records the history of their religion. The Torah includes the Talmud, which interprets Biblical commandments and deals also with many fields of knowledge. Rabbis are the spiritual leaders in Judaism.

Beginning in the 1700s, differences in ideology led some Jews to form new movements. Today in America, there are four branches of Judaism: Orthodox, Conservative, Reform, and Reconstructionist. Each movement has its own distinct institutions, rabbis, and forms of observances. Jews adhere to commandments; recite daily prayers; attend services and honor a weekly holy day (the Sabbath); observe dietary laws; hold ceremonies for circumcision, coming of age (bar and bat mitzvah), marriage, and death; and celebrate several annual holidays, among other religious observances.

Christianity arose out of Judaism in the first century A.D. Jesus Christ, a Palestinian Jew, claimed to be the Messiah, sent from God to reveal God's will and redeem the people. Jewish authorities refused to accept him as the Messiah and he was crucified as a political troublemaker. A group of his followers won converts by preaching redemption and salvation through Christ.

Christianity spread throughout the Roman Empire and struggled against persecution, but eventually prevailed throughout Europe. After some 1,000 years of solidarity, in 1054, ideological and cultural differences led to a split between Christians in eastern and western churches. In the East, believers formed the Orthodox churches, while believers in the West maintained their allegiance to the Bishop of Rome, thus forming the Roman Catholic Church.

A large group of Christians broke away from the Catholic Church in 1517 after a young monk named Martin Luther protested various doctrines and practices within the Roman Catholic Church, such as the sale of indulgences and certain sacramental practices. A period called the Reformation followed this event, during which Protestantism grew and spread throughout Europe. Protestants split into several groups and subdivided again and again, each group with its own church, leaders, and belief systems. The largest Protestant churches in America are the Baptists, Lutherans (named after the original reformer), Methodists, and Presbyterians. Most Protestants, but not all, use the Bible as a source of guidance, believe in the Holy Trinity (God the Father, God the Son, and God the Holy Spirit) and recognize the sacraments of Holy Communion and baptism.

Islam was founded in Arabia by Mohammed in the seventh century A.D. Followers of Islam are called Muslims, meaning "those who submit" (to the will of God). Islam is an extension of Judaism and Christianity, but Muslims believe the Christian Trinity is blasphemy. They acknowledge Jesus Christ as a prophet, but deny his divinity. They believe in one God, called Allah, and the Koran is their basic source of Islamic law and ritual. Religious obligations include profession of faith, prayer, almsgiving, fasting, and a pilgrimage to Mecca at least once during a Muslim's lifetime. The two major branches of Muslims are the Sunnites, or traditionalists, and the Shiites, who broke from orthodox Islam about 679 A.D. and have since split into a number of groups.

There are hundreds of religions in the world today and thousands of branches, denominations, and sects. Ideological differences within groups are likely to continue causing splits and development of new religious groups.