>>24681924
>how and why Pagan religions were conquered by Christianity
Monotheistic faiths have a nasty habit of sneaking up on polytheists by becoming one cult among many and then spreading like wildfire before turning against "idolatry". I sincerely recommend "The Final Pagan Generation" by Edward J. Watts, which examines Rome's conversion to Christianity. Specifically it talks about how as Christianity spread the Roman elite was unconcerned because it was just another eastern cult (scarcely the first time Rome had been touched by adherents of such things among the elite). And when Julian the Apostate tried to resist Christianity, the last pagans did not throw their support behind him to preserve some "pagan way of life" but rather criticised his drastic anti-Christian measures as violating the Roman way of life.
I don't have a book to recommend on how Rome's Germanic (Christian) successor kingdoms managed to convince most other pagans to convert. Military conquest did happen (e.g. Charlemagne's war with the Saxons or the Northern Crusades, about which you can read in Tyerman's "God's War: A New History of the Crusades") but many powerful eastern and northern kingdoms such as Hungary, Bulgaria, Kiev, Denmark-Norway converted voluntarily.
Adapting a universalist religion was politically advantageous for them to be accepted into Christendom and enter diplomatic relations on equal footing and to unify their populations (divided into tribes, sometimes belonging to radically different peoples such as Slavs and Magyars/Bulgars) into "nations".