Really the theology was not that good at the outset. It's a religion structured wholly around an immediately imminent eschatology. Those don't tend to stick around for very long, because: what do you do when you say the end of days is next week and Tuesday rolls around and nothing happens? Moreover, it's clear that the theology wasn't particularly elegant or straightforward, because of the sheer number of heresies and general disunity that occurred over the course of the Late Antique period. Paul and Tertullian devoted much of their time in writing letters to congregations reminding them to stop circumcising their sons and that women need to keep their heads covered. Augustine engaged in a massive, religion-establishing debate with Pelagius over predestination, and a whole subset of Christians ran around calling Jesus a demigod. The essentiality of the trinity would seem to us to be the sine qua non of the whole religion, and even that was not universally acknowledged. It wasn't until an Emperor personally convened a council to sort this stuff out, and then set about enforcing that council's findings, that any of this stuff began to be resolved. That's a far cry from the Roman pantheon's: "sacrifice a goat and you might get something good if the Gods feel like it"
The religion was very fortunate that:
1) It formed in the Roman Empire, a heavily urbanized polity with a robust and well-connected trade system
2) It benefited from a number of zealous proselytizers who also happened to be excellent rhetoricians
3) Some brilliant philosophical minds, notably Paul, Tertullian, and Augustine were able to take the existing theological groundwork and shape it into something genuinely elegant, capable of standing up to scrutiny.
These things regarding proselytizers rhetoricians and trade systems that you state are yes, are facts, though these things were available to everyone else at the onset of the first century. The fact that there were numerous heresies developing underscores growing Christianity's success within the hostile Empire.
The question being explored here is WHY should persecuted 33-300 AD era Christians have any advantage here unless they have something REALLY worthwhile to say or offer the other then existent religions could not ?
Yes it is politically incorrect to suggest that one religion has doctrines that are superior than another, yet--
why, how did Christianity succeed in its first 400 years without militant revolt or conquest?
Just what were some of its doctrines that allowed it to sustain that was not present in the polytheistic religions of Rome?
As per the article there is the social component of the the theology:
" slave and noble greeted one another as brothers in Christ"
So apparently this egalitarianism was missing from what Rome had in its religions. Slaves under Christ had the same standing as nobles.
According to tradition, Pope Clement I (term c. 92–99), Pope Pius I (158–167) and Pope Callixtus I (c. 217–222) were former slaves
--turning mercy and pity into a VIRTUE rather than a defect of character; (because one is sharing resources with those who did not earn them without return benefit)
--the mindset that love and charity must extend beyond the " boundaries of family and tribe" and the "Christian community" to those who are not a part of it
--God DOES care HOW others are treated .....(quoting the article) "the gods can be induced to exchange services for sacrifices....., the idea that God loves those who love him was entirely new.....that because God loves humanity, Christians may not please God unless they love one another was something entirely new. "
Jews had these ideals since Christianity came from Judaism, Jesus was a Jew, however, because of the necessity to "ethnically" (as per article) become a Jew when converting, it simply was too huge of a barrier for others thinking about a decision to become part of Judaism.
"Christianity first evaded and then overwhelmed the ethnic barrier that had prevented Judaism from serving as the basis for revitalization. "