During the era between 300 and 1500 CE, many religions expanded throughout Afro-Eurasia due to several reasons, but the main reasons were at least three: missionaries, trade, and empires.
This era saw the rise and expansion of three great world religions: Buddhism, Christianity, and Islam.
Buddhism: while this religion (many times considered a philosophical and spiritual system) started around the 6th century BCE, it continued to expand during the era between 300 and 1500 CE. It did so chiefly due to the work of missionaries and Buddhist merchants. Bodhidharma was a Buddhist missioner from India that, during the 6th century CE, took Buddhism to China and, eventually, expanded this religion to the rest of Eastern Asia. Another important element in this period was the existence of the Silk Road that connected Western markets in Europe and the Middle East with the markets of Eastern Asia, like India, China, Mongolia, and Korea. This commercial link allowed the exchange of ideas and beliefs, among which Buddhism played an important role due to the great number of Buddhist practitioners and missionaries that used this road. This religion grew mainly in China and Central Asia thanks to this commercial network.
Christianity: this religion started growing around the Mediterranean Sea during the 1st century CE. Thanks to the work of Christian missioners, the number of followers grew largely in the Roman Empire's territory to the point that the Roman Empire had to recognize it as a legitimate religion in 325 under Emperor Constantine, and it was established as the official religion of the empire in 380 under the Emperor Theodosius. After this event, the Christian Church acquired a strong political and military power and continued with a more aggressive expansion and defense of the Christian doctrine during the following centuries.
Islam: this religion started during the 7th century CE in the Arabian Peninsula following the teaching of the Prophet Muhammad. It rapidly expanded throughout the Middle East, North Africa and the Iberian Peninsula due to military campaigns that established the Islamic Empire. This religion also grew thanks to the labor of Muslim missionaries and merchants that used the trade routes in the Middle East and North Africa.
