I'm reading yet another of those books, written at or around 1900, roughly speaking, which contrasts ancient philosophy and religion with Christianity, and asserts Christianity is in some mysterious (pun perhaps intended) way superior, or perhaps it would be more accurate to say of superior appeal.
This particular book professes in its title that it addresses the conflict of religions in the Roman Empire, but seems to be primarily interested in Stoicism and Christianity. Only passing mentions of the great pagan cults of the early Empire, thus far.
I fear that the primary reason the author feels Stoicism "failed" while Christianity triumphed is that Stoicism is too gloomy. Relying almost entirely on quotations from Marcus Aurelius, the author asserts that Stoicism, though admirable in many respects, could only manage an attitude of weary resignation to the universe, not the joyful acceptance of it found in Christianity. Stoicism's God was abstract, while that of Christianity intensely personal; the God of the Christians was a loving father, "Abba." The prodigal son was welcomed by a wronged but loving parent who ran to embrace him.
If the author had read Epictetus (he doesn't mention him) he might feel differently. Marcus Aurelius, we know, was thought of as somehow unmanly by the more "muscular" Christian apologists such as Chesterton, who in touting Christianity had a disturbing tendency to refer to his nanny and fairy tales, and C.S. Lewis, who decided to end his charming tales of Narnia with the annihilation of his heroes and heroines in a train accident, and may or may not have had a nanny.
But the author, however, does a good job in referencing the simplicity of the early Christian message and the fact that it was not directed to those who could and did frequent philosophers and read their works. The message was addressed to the "common" and contrasted them favorably with the Pharisee and the rich man. Although it taught many of the ideas accepted by philosophy, and indeed the Stoics (e.g. that God permeates the universe and we are part of him, or he is in us) it did so in such a fashion that they could be accepted by those without philosophical training. The philosophical Christians came later, bringing with them and incorporating into Christianity what they learned from Plato, Aristotle, Seneca, etc., just as the many Christian converts throughout the Empire brought with them pagan religious practices and customs of their homelands. Christianity thus managed to incorporate what all found attractive in religion or religious thought by most of the citizens of the empire. As a religion, it assimilated, and continued to assimilate for some centuries to come. It was a remarkable achievement.
Gradually, Christianity became organized. As it became the favorite of the emperors, and bishops were granted imperium in various respects, including in some cases high positions within the empire, it took on the aspects of the empire, even became the empire in some sense where the empire failed.
The attractiveness and simplicity of the message functioned to make it popular in a time when people were seeking salvation and eternal life; the pagan cults which did the same were obscure and occult, and their initiates were often sworn to secrecy. Christianity's appeal was universal.
With organization, though, came power and power corrupts, as we know, and has I think corrupted for many centuries. It had magic, but doesn't seem to have it any longer. There are and have been those who purport to bring back the ancient simplicity of its message, but this likely is no longer possible.
And so perhaps organized Christianity finds itself in much the same position as the Empire found itself at the time Christianity first appeared and gradually spread. Still well organized, powerful, following ancient customs and rituals, but lacking in the ability to inspire. I wonder what will take its place.