Indeed. It was all over the place, but in India, Iran, China, etc. Didn't last more than a century or two in China, but the Nestorians didn't have the numbers for missionary work on a large scale. Neither did the Catholics (or some other Christian group) when the Mongols asked for some missionaries.Ten percent is the lowest bound; other sources put it as high as thirty. Even if it was only ten, though, going from zero to ten in the face of official hostility and persecution is still pretty good, and whilst being made the state religion was undoubtedly a major boost there's no reason to suppose it would be "screwed" if this didn't happen.
And yah, what would make so many groups come together? Christianity was universal and not based upon profession, class, ethnicity, region, etc. And, most importantly, it was not about keeping secrets. You went out of your way to tell people about your faith and anyone with knowledge of the doctrine, Gospels, etc were explicitly told not to keep it secret. If we think of all the famous secondary religions in the Roman Empire that people bring up elsewhere as alternatives to Christianity often have 'mysteries' as part of their full classification... Yah.