Not that I believe that "Paul" was an actual person
Well, that's an unusual position, and one not supported by the scholarship. I disagree, but consider the topic to be somewhat trivial. If you wish to start a new thread to discuss the topic, I will present my evidence for accepting the historicity of Paul, but if the topic is unimportant to you, I am certainly willing to agree to disagree.
early Christianity was a mystery religion.
Sort of. Early Christianity shares _some_, but not all, attributes with Mystery Religions. The largest difference was the lack of a need to pay for initiation, and the lack of secrecy.
Some people certainly did convert of their own free will. But not most and most certainly not all.
By the time of Constantine's death, Christianity was a significant percentage of the population--though of course, there is no way to know what percentage that constitutes. Yes, I doubt they were a majority, but we simply do not have the evidence to discern that, one way or another. But even before the "official"persecutions began around 350, Christianity was a popular religion within the Empire.
The official church version of the conversion of Europe, starting from Classical antiquity, is almost a complete lie. The masses did not gleefully come running into the embrace of the church after Constantine legalized Catholicism.
Saint Frank, you have to have evidence to back that assertion, else it is simply an unfounded assertion. One substantial piece of evidence against your assertion is the presence of the unofficial persecution against non-Christians, even before the Edict: the mobs and the riots had to have a sufficient population to support them. Another indicator is the failure of Paganism to re-assert itself during the reign of Julian, or of the three emperors who followed him.
By the time of Julian, Christianity had enough of a percentage in the population to survive the lack of preference of the reign of Julian, Jovian, Valentian, and Valens.
To be sure, the Christians behaved in an absolutely vile manner towards those they opposed. The fact that they behaved towards the Pagans exactly as the Pagans had behaved towards them (rather than, as their scriptures command, to respond with submission) is, for me, a good indication that their religion is every bit as false as all the other religions out there. But we cannot discount their numbers: the evidence available forbids such a conclusion.