I've been thinking about this for a bit and we talk a lot about the events leading up to the adoption of Christianity as a state religion and we talk about how Christianity was able to integrate itself into local cultural traditions. We sometimes also talk about Mandaeanism as an early competitor to Christianity.

Maybe I haven't looked through the FAQs thoroughly enough, but the focus seems to be more on the adoption of christianity, rather than the abandonment of paganism. Having said that, I do remember someone suggesting that converting religion (or just praying to a different god to see if he answered your prayers better) was less stigmatised amongst pagans. In which case, was it inevitable that something might come along which did stigmatise apostasy, which could displace paganism?