Final answer:
Christianity gained popularity due to its doctrinal simplicity that was easily understandable and inclusive, appealing to a broad audience, and its active missionary activities that aggressively spread the religion's messages and invited conversions.
Step-by-step explanation:
The popularity of Christianity among both Jews and non-Jews can be attributed to several reasons, but two of the most significant are its doctrinal simplicity and its active missionary activities. Christianity's straightforward teachings on spiritual equality and compassion resonated with people across various social strata, challenging the rigid Roman class system.
Furthermore, the religion's inclusivity, widened by figures like St. Paul, allowed anyone to embrace the faith, not just those of Jewish ancestry. Its successful spread was further aided by concerted efforts of its leaders to proselytize and construct a unified Christian identity, driving conversions and entrenching the faith across large regions.