Looking at the history of missions allows us an opportunity to see what might have gone wrong. Why was the gospel lost in Asia and Africa after those early years? The Mongols of Asia and the Muslims invaded much of the area, but we believe that syncretism was the main problem.
Syncretism means mixing the gospel with local perspectives of the culture. This happened a lot in the early African church with the adoption of the worship of Mary replacing Isis, for example. It also happened in most places in Asia where the Nestorians preached.
The Nestorians, being more humanistic, celebrated novel gospel interpretations in new cultures. They also went so far away geographically in missions that their “supply lines” grew thin. They were cut off from the main theological development in other sectors of the church.
For example, Bishop Qing Qing of China wrote in 780AD, comparing the gospel to Buddhist Confucian karma. Qing claimed that we return to our original good nature by works. In his Sutra of Returning to Your Original Nature, he claimed:
Christ says: From goodness in past lives, people come to this religion and through the faith they find Happiness…Simon (Peter) know this: You ask me about the Triumphant Law. What your ancestors have done bears fruit in you; their karma finds its outcome in you. (Cited from Martin Palmer).
Here is a Christian Bishop making a Christian theology for China 1,227 years ago. He believed in reincarnation. He misquoted Jesus’ words to Peter in Matthew 16. He claimed that the original nature of each man when born is good. He said we restore this original goodness through our good works. He said that Simon Peter was blessed, not by the free grace of his heavenly Father (Matt 16:17), but by his own good works. This is Buddhism, not Christianity. This is Nestorian humanism.
When Christianity is no different from local religions what is the point of it? It is absorbed into traditional religion like a chameleon lizard and disappears into the fabric of its environment. This is one of the greatest hazards of international mission. This was the issue Paul addressed in Galatians and was the main problem addressed in the book of Hebrews: the gospel was being mixed with Jewish culture. Paul said that the gospel was then lost.
We also must consider the importance of church mission influencing government. In countries where Christianity did not impact and influence government it eventually lost ground. In Ethiopia, Armenia and Europe it held its ground. It is true that the church was corrupted by government in Europe, but at least there was something there for the Reformation to correct.
The conversion of the Roman Emperor Constantine in the West in the early 4th Century brought relief to the Christians from the Roman persecution there. Persecution of Christians had become Roman policy, but Constantine made the Roman Empire Christian.
But Constantine’s coming led to a great increase of persecution against Christians in the East. The East and West were enemies then and Persia saw Christians in Persia as potential allies of Constantine in the West and spies. So the Western church celebrates Constantine, but maybe not the Eastern Church.