Background
Humans have always expressed a need to understand natural phenomenon and to answer questions regarding their way of life and what happens once they die. These needs resulted in the development of a variety of religions and philosophies that can be found throughout the world today.  Some of these religions remained localized in the region they developed in, while others spread extensively throughout the world.

Buddhism
Buddhism spread from India to different parts of Asia, including China, where it became very prominent.  This occurred during the Maurya Dynasty in India when Emperor Asoka converted to Buddhism and then sent out missionaries.  Buddhism was also spread along the extensive trade routes between India and the rest of Asia.

Judaism
Judaism was spread when the Romans forced the Jewish population in Palestine into exile in 135 CE.  This is known as the Jewish Diaspora, or scattering of peoples.  Jewish exiles spread throughout Europe, the Middle East, and Russia.  They usually lived in very small communities and worked hard to preserve their religion and cultural heritage.

Christianity
Christianity first spread through the work of missionaries.  It attracted a lot of followers, despite the Roman Empire's persecution of the religion.  In 313 CE, the Roman Emperor Constantine ended the persecution of Christians with the Edict of Milan.  About 80 years later, the Emperor Theodosius made Christianity the official religion of the Roman Empire. After the fall of the western Roman empire, Christianity continued to spread through the work of missionaries along the trade routes of Europe.  During the Age of Exploration, Christian missionary religious orders, such as the Society of Jesus, spread the religion around the world.

Early Spread of Christianity

Islam
Islam began in the Arabian Peninsula in the early 7th century CE.  It quickly spread throughout the Middle East before moving across North Africa, and into Spain and Sicily.  By the 13th century, Islam had spread across India and Southeast Asia.  The reasons for the success of Islam, and the expansion of its empire, can be attributed to the strength of the Arab armies, the use of a common language, and fair treatment of conquered peoples.

Arab armies were able to quickly conquer territory through the use of advanced tactics and the employment of horse and camel cavalry.  Islamic rulers were very tolerant of conquered peoples, and welcomed conversion to the Islamic faith.  All Muslims must learn Arabic, so they can read the Qur'an, the Islamic holy book.  This common language helped to unite many different ethnic groups within the Islamic empire.  It also made possible the easy exchange of knowledge and ideas.