Correct - the christianity that eventually gained global success was hugely different to the christianity of the earliest adherents. Literally beyond belief as those early christian communities firmly believed that the end of times would have happened centuries before Constantine was even born.
I wonder then why you have inflicted your theory about how the later Christian church gains and retains members on the early church.
Certainly and eventually nominal, socially incentivised Christianity became more of a thing where as in the growth period repentance and personal encounter would have driven the faith.
You say Christianity failed to grab the locals.So what? Judaism was very much the religion for locals and had, I would imagine already embraced nominality and birth qualifications for membership in the way Christianity never did.
On another matter the religious magisterium of the Roman Empire was never as big as the post Constantine Catholic magisterium and as such I think Christianity was a bigger deal than you make out.
What I find fascinating is Atheist detractors of today seem to think the church was influential enough to derail and sap the muscularity and pud out of the Roman Empire , which presumably would have experienced some kind of enlightenment through culture and reason on it’s own and on the other hand influential enough to have changed the policy on slavery which seems to go against your small, fairly invisible sect Hypothesis.
I still get the whiff of argumentum ad populum from your efforts.