Ironically, the traits that made Christianity so successful were the very ones that pagan Romans found incomprehensible, repulsive, alien, and subversive. To accept only one god was unfathomable to a polytheist. Interestingly, some Romans wanted to incorporate Jesus into their pantheon of deities. When Pontius Pilot reported to the Emperor Tiberius that the Jewish Messiah (Jesus) was unjustly killed it was, "...Tiberius, who avowed his contempt for all religion, immediately conceived the design of placing the Jewish Messiah among the gods of Rome". Wouldn't that have resulted in a different future for Christianity?
In any case, since Christians rejected worship of any other god but theirs, this put them on a collision course with Roman culture and mores. Interestingly, many eminent philosophers (the intellectuals of their day) acknowledged the wisdom of a single god. However, they could not accept the concept when it was presented to them by a group of ragtag disciples from distant Judea. In addition, many Romans found the austere, disengaged lifestyle of Christians to be haughty and traitorous to the ideals of the Roman Empire.
Gibbon tells us that not as many Christians became martyrs as we are lead to believe. Examining the records, Gibbon says that far more Christians have been killed in the religious wars of Europe than were ever persecuted by the Romans. However, there appears to be no shortage of Christians who were willing to embrace martyrdom. In fact, many seemed far too eager to leave their temporal existence for the Kingdom of Heaven than logic would dictate. To avoid martyrdom was quite easy as it did not require a "confession" only a "denial" and a small offering to one of the pagan gods. Gibbon says that many of the poor and humble Christians had very little to look forward to in this life. This made martyrdom and eternal bliss in Heaven an attractive escape and many would in fact rush toward death - even antagonizing the beasts in the Colosseum to speed along their demise.