The first Chinese pilgrim to actually reach India and return with a knowledge of Buddhism was Fa Xian (377-422), a monk who travelled the southern route in 399, through Dunhuang and Khotan and over the Himalayas to India. He studied Buddhism under various Indian masters in Benares, Gandhara and Ceylon (Sri Lanka), and went as far as Sumatra and Java in Indonesia; altogether he visited over 30 countries, returning to China in 414 via the sea route. The Buddhist monk, Xuan Zang (600-664), is perhaps the most well known of all Chinese travellers on the Silk Road, and one of the four great translators of Buddhist texts. His lasting fame is primarily due to the humorous 16th century novel, Journey to the West (also known as Monkey), a fictional account of his pilgrimage that includes the various escapades of an odd assortment of characters who accompany the monk on his journey.
Xuan Zang left Chang’an in 629 and travelled along the northern Silk Road to Turpan, Kucha, then onto Tashkent, Samarkand and Bactria, over the Hindu Kush to Gandhara and eventually further south to Sri Lanka. He studied Mahayana Buddhism, particularly the Yogachara school, at various monasteries for 14 years and became a renowned scholar, winning many debates against Hinayana Buddhist scholars. He returned to China in 645 via the southern Silk Road and wrote Records of the Western Regions, an excellent account of his travels and the state of the Buddhism in the seventh century. With a disciple he co-founded the Fa Xiang school, the Chinese form of Yogachara, which was popular during the Tang dynasty. The central tenet in this belief is that the external world is a product of our consciousness, things exist only as far as they exist in our minds, and nirvana (Buddhahood) is achieved after working through several complex levels of spiritual development and detachment. The Fa Xiang school denies that Buddhahood is possible for everyone, in direct opposition to other Mahayana schools, and it actually contributed to the latter’s decline after the Tang dynasty. Xuan Zang translated over 75 Sanskrit works into Chinese, and translated the teaching’s of the Taoist philosopher, Laozi, into Sanskrit as well. His translations were known for their high literary content and he was instrumental in creating and extensive Buddhist vocabulary in Chinese. The Big Goose Pagoda in Xian was built to house the 520 Mahayana and Hinayana texts and various relics that he brought back, and this was where he worked for the remainder of his life, translating sutras.
The religious of Manicheanism and Nestorian Christianity were also introduced, accepted the assimilated along the Silk Road, although neither reached the popularity enjoyed by Buddhism. Manicheanism was started by Manes of Persia in the third century BC and is a religion based on the opposing principles of light and dark (spirit and flesh). Followers of Manicheanism, persecuted by the Sassanian kings in the third to sixth centuries AD, began arriving in Central Asia and flourished during the Sui (581-618) and Tang dynasties. Until the recent discovery of Manichean libraries and wall paintings at Karakhoja (near Turpan), little was known of this religious sect, believed by most scholars to have no literature or art. It sustained a substantial following into the tenth century, but then quickly disappeared with the advent of Islam in the West and Buddhism in the East. However, Manicheanism survived in southeastern China until to 17th century.
One of the essential beliefs of Nestorian Christianity is that Christ is fully human as well as fully divine, both natures being complete side by side. This belief was condemned in 431 by the Council of Ephesus and hence forbidden in the Roman Empire. The Independent Church of the East, based in Seleucia-Ctesiphon near today’s Baghdad, retained this belief. Unable to expand westwards, the Persian Church sent its missionaries east towards China in the seventh century. Nestorian manuscripts were discovered in the Turpan and Khotan regions and Marco Polo found thriving Nestorian communities in cities along the Silk Road as late as the 13th century, even though all foreign religions had been heavily persecuted in 843-845. eventually, under pressure from Islam these religions disappeared from northwestern China.
From Book "The Silk Road Xi'an to Kashgar"-Judy Bonavia