This is a difficult thing to talk about because (I believe) historians
have not yet seen through to the bottom of events circa the beginning of
the common era.
Here's my potted version: Round about 600 b.c.e. onwards, as a result
of the culture clash caused by the Alexandrian empire and improved
communications, the old religious forms, based as they were on tribal
certainties, began to be doubted, and people started to roll their own
religions. These were largely of a Gnostic/Theurgic/Mysteries form, in
which each individual for him or herself faces the mystery of the
universe alone and nekkid (without the doubtful tribal intercessors and
cultural filters). This (along with the usual Greek and Roman philosophical
sources) is the true origin of the individualistic/libertarian meme.
By the time of Constantine, these forms of Gnosis had so burgeoned and
infected the Roman Empire (supplanting the old Roman state religion) that
some Romans feared it was in danger of dissolution from within. (Of
course the real reason for the dissolution of the Roman Empire was the
decay of Republicanism, but that's another story!) So Constantine
either co-opted or created an eclectic form of Gnosticism/Theurgy as the
new Roman State religion.
If he co-opted, he must have found an already eclectic and pretty
corrupt form of Gnosticism/Theurgy, and simply promised its leaders
sweets ("We will help you destroy your ideological enemies if you allow
us to make your religion, suitably altered, a state religion").
If he created, he must have gotten some sort of council of corrupt and
dissatisfied power-lusting leaders of various sects together, and paid
them to create an eclectic form.
Hence, there *are* minute amounts of truth to be found in Christist
doctrine, but these are pretty subtle, and are only teaseable-out by
intelligent, charitable interpretation, and are more than outweighed by
those aspects that make Christism such a suitable religion for dominators.
(Note: I follow prof. G.A.Wells' notion that 'Christ' never existed, and
is a mythical figure who became historical, somewhat in the manner of
William Tell. This would tend to support the 'co-opted sect' version of
the above history.)
It is also of interest to note that 'Christian' may have been a sort of
general term denoting the burgeoning Gnostic/Theurgic sects. 'Chrestoi'
(meaning 'anointed', as in anointing a king) was a common appelation
found on the tombs of members of various Gnostic/Theurgic/ Mysteries
sects round about the period in question, IN SOME CASES ON TOMBS B.C.E..
It meant something like 'justified', as in 'soul weighed and not found
wanting' (compare the Egyptian Book of the Dead).
I think it will take a good many years of non-Christist research, and
not a few more lucky discoveries like the Dead Sea Scrolls and Nag
Hammadi, before we even begin to understand what happened then, and the
terrible darkness that fell over the earth with the rise of Christism as
a result of what happened then.