How can Christianity still arise like OTL, but never become a majority religion, and still survive to the present day as a small religion like Judaism or Jainism or something? Preferably not simply by Judaizing itself until its just some messianic Jewish sect, and at least somewhat recognizable as OTL Christianity (though some Arian-like beliefs would be fine). Note that they should never become a major religion, so nothing like Islam conquers Europe and converts 90%. Having one or two small regions like Ethiopia or Armenia become majority Christian is good, just not swathes of continents.
My idea (that I tried in my early timeline attempt) was to make Christianity more radical, like Jainism is. Could that work, or was the Roman world not conductive to small radical religions like that and it would be wiped out?