When it comes to delineating the difference between religion and cult, I always think of Chomsky’s remarks about the difference between a language and a dialect:
A religion is a cult with an army.
vs
A language is a dialect with an army.

This doesn’t have to mean anything about actual armies, btw, this is just the point that the numbers determine what is a ‘language’ vs what is a ‘dialect’, and so too with religion. The cult of Isis in the roman empire shared many commonalities with early Christianity, and both were treated as cults. Both are mystery religions (in the technical sense of ‘mystery religion’); both afforded Roman women equality in a way they were not permitted in ‘official’ religions— one has survived. The believers will say it is because one is ‘the real thing’, but in reality, it is always about the numbers. Just as the victors write the history books, so too does that apply in the history of religion books.