There are many traditions about early apostles and their travels. Thomas is said to have preached in India. We do not know this for certain, but it is certain that the church existed in India in the 1st Century. Secular sources show Christians to be living as far east as northern Afghanistan by 196AD, so we know that they were established there before that date.
Christians travelled very early on the trade routes to China and beyond and to India and the sub-continent region. By the 5th Century the Asian Nestorian group was very strong and missions orientated. From Selucia-Ctesphon in Persia (modern Iran) by the middle of the 6th Century mission groups went to Egypt, Syria, Arabia, Mesopotamia, Persia, India, Ceylon, China and Mongolia.
It is possible that Iran, in its past, sent out more Christian missionaries than any other nation since. They took the gospel all over the world. By the 500’s the church in China was strong. By the 700’s Chinese bishops were writing their own theology books. There is archaeological evidence of the gospel in Japan, Indonesia and the Philippines by the 700’sAD. Historian John Stewart states:
Whole peoples with their rulers had become Christians and it seems certain that there were few places in the whole of Asia that were not reached at some time or other…and in the 11th Century (the Asian church) is said to have outnumbered the Greek and Roman churches combined.
From the Pacific Ocean in the East to the Mediterranean in the West; from the Black Sea and Siberia to the Indian Ocean and Arabian Sea, Assyrian missions were working. Asia Minor, Cyprus, Egypt, Palestine, Mesopotamia, Arabia, Persia, Afghanistan, India, China, Japan, Mongolia, Manchuria and Turkistan…all had missions where the gospel was taught by zealous workers of the Assyrian Church of the East...
Before the Islamic conquests of the 7th Century the largest Arab kingdoms were Christian. When Paul was saved he went into Arabia, not to meditate in deserts, but to fellowship with Christians. Persia had a bishopric in Qatar. Yemen in the far south west was evangelised in the mid 300’s by Theophilus on route to India.
Persecutions in Persia drove many Christians to evangelise Arabia. Monasteries were built along the Arab side of the Gulf. The Persian Synod of 410 had bishops from Qatar and Bahrain. In 225 settlers found a group of native Christians near Babylon. Nestorians formed Christian communities (umma) between normally aggressive tribal groups of Arabia. Some of these Christian communities survived long into the Islamic era.