Early Christianity till the 6th century

I just listened (thanks to Librivox) to about half of an old book, titled History Of The Christian Church During The First Six Centuries.

The book documents Christianities leaders, branches and debates up to around 600 AD. The book documents the growth of the church. It also explains how they changed some practices -- e.g. did not insist on circumcision -- in order to make conversion more palatable to gentiles.

However, the author did not explain why those gentiles (or other Jews) would switch to the Christian sect of Judaism. After Constantine moved the Roman empire to Christianity around 300AD, the rise of the religion can be explained by political sponsorship. However, I did not find what I was looking for: i.e., an explanation of the motivation (intellectual or other) of people who adopted Christianity in the first three centuries.

Three of the four largest religions -- Christianity, Buddhism and Islam -- spread very widely. I'm curious about how religions spread without political sponsorship. Why did Christianity grow in the first century or so? Why did Buddhism spread in China? (Political sponsorship was the key to its spread in India, and it faded when that sponsorship ended.) Perhaps a lot of Islam's success may be explained by early political sponsorship; but, here too, its spread to Indonesia and to the south-west coast of India seems to have been via evangelism.

Older faiths just "were". Conquerors would convert new kingdoms to their faith, but there seems to have been little evangelism in older religions like Hinduism, Zoroastrianism, Mithraism or the old Greek or Roman practices. Could a large part of the success be explained by the evangelical impulse itself? Could it be as simple as that? I.e. that only a few religions made an organized effort to spread their faith, and that the three major ones are among the few that did?

Apart from the desire to evangelize, perhaps these religions owe much of their spread to the fact that they developed some types of methods, institutions and networks that were geared toward evangelism.

Looking at the original theology of these three religions, I cannot find anything radically unique to set them apart from other sects of their time. So, though I don't know enough to rule out that they really did fill some intellectual need that was not filled by their successors, I suspect that theology is only a minor factor.

One tidbit that I found interesting was that the author identifies two hold-outs against Christianity after the Roman emperors had adopted it officially. The first was rural areas, particularly if they were isolated. No surprise there. The other holdout was the scholars in universities, who continued to dismiss Christian mysticism for a while after others had converted.

One negative with this book is that the author hints at mystical explanations for some historical events; but, it is not enough to distract.

All in all, it was a mildly interesting book. It helped make the period more real to me, but I'm still left with the questions I began with.