Faith Without Words

Spawned from comments in yesterday’s post.

I wonder if modern Christianity or Islam really could succeed in an illiterate society.

Consider an isolated and illiterate people group. A single external missionary is allowed to evangelize to this group with any resources available (including holy texts for reference), but the missionary is not allowed to teach the people to read. The missionary continues until all the population (or a sufficient percentage) become converts, after which the missionary departs and the people remain in isolation.

As time progressed, would oral tradition be sufficient to sustain the religious tradition so that it remains doctrinally consistent with the missionary’s teachings? If a religion is genuinely a fundamental truth, then there should be a low risk of contamination to the point of contradiction. But at the same time, without a holy text as reference it would be difficult to measure any doctrinal drift.