The following numbers, although approximate, enlighten nonetheless: around 30 A.D., Europe was 100% pagan. The only possible exceptions were tiny Jewish communities that might have existed in Rome and in some of the Greek city-states. Numerically, these would have been insignificant, if they existed at all; we know that, a few decades later, they did exist.
This means that all of continental Europe, from Spain to Finland, from Italy to Greece, was dominated by a belief system which featured polytheism, human sacrifices, and - in its primitive stages - ritual orgies. This pre-religious quasi-spirituality (for it was neither fully, but perhaps both partially) consisted of myth and magic. Myth is the attempt to explain; magic is the attempt to control. Mythological explanations were offered for the weather, for childlessness, and for military victories or losses. Magic tried to manipulate harvests, human fertility, and the outcomes of battles. Lacking was any sense of personal relationship between the human and the deity.
This, then, was mindset which dominated the area.
By 400 A.D., the majority of the European landmass will be inhabited by populations which contain a significant minority of Christians; some areas will even have a majority of Christians. By 800 A.D., the composition of all of Europe will be approximately 45% Christian and 10% Jewish; the remaining 45% will claim to be Christian. Paganism will be essentially gone; possibly, tiny groups of Druids or others remained for a few more decades in hiding.
Given that paganism had dominated the continent (as well as most of the world) for around five thousand years, it vanished with shocking speed. Although a few centuries may seem like a long time to you and me, it's a mere instant in the grand scheme of world history.
Two questions remain to be asked: Why did people so easily relinquish their old belief system and embrace a new one? And what was the net effect of this change?
To the first question, we may note that ancient paganism had little with which to endear itself to practitioner, and so it would be easy for those people to let go of it. It lacked any sense of personal bond to the gods worshipped, and lacked concepts of forgiveness, comfort, and charity. It encouraged a sense of manipulation along multiple vectors - humans manipulating deities, deities manipulating humans, humans manipulating each other, and even deities manipulating each other. It nudged cultures toward desperation and fear; it spoke of gods who behave erratically, unreliably, and even hostilely toward humans.
By contrast, the Judeo-Christian influence spoke of hope, friendship, and mutual aid. It encouraged humans to accept the unalterable facts of existence, rather than hope for a magical change. It recognized the limits of human knowledge and reason, rather than inventing mythological explanations for those things which lie beyond human power; it revealed a Deity who liked humans and desired friendship with them.
The net effect of polytheism's decline was manifold: most obviously, human sacrifice was ended. Beyond that, there was a change in the very idea of what it meant to be human: every human life became seen as valuable and worthy of respect. The buying and selling of people, whether in slavery or in marriage, ended; women were given a voice in their own lives and decisions. Torture was considered inappropriate, and a conflict of ideas was viewed as an opportunity for a healthy debate, not a physical conflict.
Did European culture live up to these noble ideas which were introduced by the Judeo-Christian tradition? Not always. There are glaring examples in which the Europeans failed, at certain times, to respect human rights. But there were also times at which they did the right thing: times at which they respected the dignity of the individual. And this set them apart from what they had been a few hundred years earlier - significant progress - and it also set them apart from the other cultures of the world.