Christianity as an independent religion took several hundred years to evolve from a Messianic cult of Judaism. Its structure and dynamics of conversion were significantly different after it became the preferred and then official religion of the Roman Empire.

Outside of the Empire conversion became intertwined with conquest: a major shift from it being a communal religion of the dispossessed as mentioned in other comments

Secondly most of the comments relate to the experience within the core of the empire. The situation outside of the Empire such as Ireland, never conquered by Rome, is different. What little evidence we have suggests an introduction of early Christianity (before Constantine) through farm monastic settlements that were superseded by later post Constantine Christianity based on church territories, politically allied with secular powers.

However the dynamics as to how and why the early introductions happened is speculative because of lack of any definitive evidence. Attempts have been made to associate it with the cultural character of the peoples of Ireland but these are unconvincing

Hinduism, as far as my limited understanding goes, with its background and status of Vedic literature was successful through the slow absorption of fragmented local beliefs. For instance local deities were claimed to be avatars of Shiva and thus became part of the Hindu religious family