A favourite theme of the late, great Geoffrey Ashe was that the transition between paganism and Christianity went smoothly and peacefully in Britain.
Unlike on the European continent; the British pagans (whether Druidic or Roman in their religion) did not seem to persecute the new Christian religion; and later-on the Christians did not persecute the pagans when they got the upper hand.
What seems to have happened is that the Christians took-over the sacred pagan sites, and 'repurposed' or rebuilt them as churches; while the pagan Gods were replaced with Jesus, Mary, and the Saints on the basis of analogous religious functions.
(Most famously; the pagan British goddess Brigid, was replaced by the Irish Saint Brigid.)
As well as its socio-political significance; this is theologically interesting; because it suggests that there is no fundamental conflict between paganism and Christianity; that - somehow or another - people could move from pagan to Christian without major spiritual or societal upheaval.
I think this gives us a clue to the essence of Christianity; or, more exactly, what distinguishes it from paganism.
What the smooth-transition tells us on the one side, is that (despite what so many people have said, and what is still asserted) there was not much to distinguish paganism and Christianity in terms of morality and lifestyle.
The everyday and societal practice of paganism and Christianity don't seem to have been very different.
What is very different between paganism and Christianity, is what happens after death!
It seems to me that the Big Message of Christian missionaries; the "unique selling point' that Christians had to offer over and above anything the pagans said; was the prospect of resurrected eternal life in Heaven.
Whereas the pagan religions could point at either some kind of afterlife life as depersonalized spirits - in an underworld or maybe as ghosts lurking in this world; or else some kind of reincarnation into the same kind of life all-over-again but as a different person...
Christians came along with their account of Jesus Christ who died and rose again and ascended to Heaven; and who offered the same possibility to those who would follow him.
And this prospect apparently appealed greatly as a possibility superior to anything in paganism.
I think it would have been obvious to ancient Britons, as it was later to the Anglo-Saxons and Norsemen; that what Christians offered was superior if it was true.
But how could people know it was true - above and beyond trusting the historical stories of the missionaries?
One form of validation was miracles: when the missionaries were Saints who could perform miracles, then this validated their claims, because it proved they had a link to the divine.
But a second, and probably more widespread, form of experiential proof was by participation in the Mass, the Eucharist, Holy Communion.
Following-up an insight from Philip K Dick; I think we can imagine that Men, at that earlier stage in the development of consciousness, would spontaneously, passively, overwhelmingly experience participation in the Mass as a literal re-living of Jesus's death and resurrection.
In the Mass; Jesus died and came to life, and was actually-present here-and-now to those participating.
This (or something spiritually analogous) would surely have been a compelling validation of the actuality of what Jesus offered.
In sum; I think the conversion from Paganism to Christianity as it was actually experienced by people in the early centuries AD (people, it should be noted, whose consciousness was significantly different from you and me) was essentially very simple, which was why it could be very quick - and why mass-conversions, and even mandatory conversion, made sense at the time.
It was an expression of the desire for resurrection after death, as preferable-to/ better-than anything paganism could offer.
And the method of achieving this desired goal, was to be admitted to the community who ritually re-enacted Jesus's death and resurrection, such that he became actually present to the believer.
Note added: This post comes after a whole bunch of earlier posts in which - as a result of reading the Fourth Gospel as the primary and most authoritative source about Jesus's teaching - I became increasingly convinced that the core message of Christianity (i.e. the offer of resurrected eternal life in Heaven) had become de-emphasized and somewhat buried throughout the history of the Christian churches. In my opinion; the advent of Mormonism from 1830 was, to a significant extent, made possible by Joseph Smith's "re-discovery" of resurrection as the core promise of Christianity. Mormonism also brought a completely new and fundamentally different set of fundamental metaphysical theological assumptions concerning reality as pluralistic, developmental etc. But I believe that the main appeal of the new type of Christianity in its early decades was its clarity-about, and focus-upon, post-mortal life - treated very 'realistically' and as something that could (with certain conditions) confidently be anticipated - and with potential for continuation of loving mortal relationships.
