Quote:Simple explanation: Constantine needed the money to pay off his supporters.
Less simple explanation: Constantine
genuinely was a committed Christian! ;-)
Actually it was probably a bit of both. He did need money - less to pay off his supporters than to finance the building of Constantinople and to stabilise the economy by injecting large quantities of good quality gold and silver coinage into the system. But many of his anti-pagan measures had no financial incentive, and seemed motivated solely by personal belief. Constantine wanted a single unified religion for the empire, with himself at its head - the schisms and heresies in the church seem to have pained and confused him (his own version of Christianity being apparently rather unsophisticated!). His limited measures against 'paganism' probably stemmed from the same source.
Quote:Tax exemptions and preference for civic posts were powerful incentives for the aristocracy to switch.
The senatorial aristocracy were already exempt from most taxation anyway - they paid a minimal land tax, but were free of extraordinary tribute taxes and sordida munera, and benefitted from regular tax remissions. When Constantine took Rome in 312 he was careful to avoid penalties against the senate, and confirmed most magistrates in their positions - the message was apparently that business would continue as usual, and the rich would be left in peace with their fabulous wealth and privilege.
Competition for civil posts mainly aided the lower decurial class, many of whom rose to high rank and gained senatorial status in a 'new aristocracy', created by and loyal to Constantine himself. Christianity definitely helped there - a good example would be Flavius Ablabius, Constantine's very able Praetorian Praefect, who was an early convert.
There's a difference, though, between conversion to Christianity and 'Christianisation' - the latter may have been a gradual process of change over the course of a couple of generations, and meant that, while still ostensibly followers of traditional religion, many people would have found the codes and rituals of their lives becoming Christian anyway, and adapted to fit in with them. (Salzman's
The Making of a Christian Aristocracy is good on this process).
Quote:A more difficult question, IMHO, is how he convinced the military to switch from Mithrasism to Christianity... The concept of killing in the name of the god of peace is rather hard to get one's head around.
Mithraism was never universal in the army, and may always have been a minority religion (although you wouldn't know it from various fictional portrayals!). By the late third century it was in decline anyway - mithraea were still in use and altars being dedicated (by the tetrarchs at Carnuntum, among others), but Sol Invictis and Jupiter Dolichenus seem to have been far more popular with the army c.300, and possibly aspects of Mithraism had merged with a more public cult of Sol (but like most mystery religion questions, this is a bit... mysterious!).
As for Christianity - it's probably much the same process of 'Christianisation'. The soldiers may have remained effectively pagan, but the rituals that governed their lives became steadily attuned to Christianity. Besides which, Constantine had (by accident or design) very effectively 'rebranded' the religion anyway - the 'saving sign' of Christ had led him to victory in the civil wars against Maxentius and Licinius, 'by this sign you will be victor' and so on. Clearly this was the will of heaven!
Soldiers were very practical people, and a religion that guaranteed glory on the battlefield couldn't be denied. By c.324, Christianity was no longer a religion of pacifism, but of military victory.