Why Early Christianity gained adepts so quickly and, to some point of history, they outnumbered pagans? The nature of the apparently "strongest" point of Christianity against ascendant religions it’s related with the different conceptions about what happens after death and how to reach the “Eternal Glory”.
The spiritual aspects of Indo-European societies were centered on the notion of duty, honor, heroism, sacrifice, asceticism and effort, especially on the battlefield. Those who were brave and were killed in combat are elevated to the Divine category and they become worthy of worship.
Thus, Indo-European religions conceived two main ways to reach the “Eternal life”: The "path of the gods" ("to remain for glory"), and the "path of the fathers" ("to survive by offspring"), which entails the absence of survival (Oblivion) for those who left behind no deeds, no memories, no posterity, no offspring, and also excludes individual survival.
This Indo-European conception was in some way profoundly unsatisfactory and distressing to some kind of mediocre individuals who either were unable to gain glory on their own merits or their selfishness could not conceive their own existence without their own individual survival. This deceptive promise of "eternal salvation" regardless you were worthy or not according Indo-European worldview was finally offered by the various mystery cults that appeared later (Dionysus, Mithraism, Orphism, Pythagoreanism, Platonism and Neoplatonism), which later, during the final period, they ended up competing against the main mass religions, foreigners to Indo-European world specially Christianity (but also Judaism, Buddhism and Islam) thus they found in them an alternative that appeased their anguish to death.
These "religions of salvation" were extremely popular because such a promise was not associated with merit, success or heroism in battle, but with a simple moral behavior (a distorted one indeed), and even in some cases, just by believing, so the world of the afterlife became not only something more accessible for anyone, but also in something that lost its mythic-symbolic meaning and acquired a literal meaning. The Easy Heaven was very tempting to the masses of lower classes. This was the main cause of the first massive
contagions conversions to christ-insanity in Europe and that gradually led to the dissolution of the Indo-European world.