Which of the following is the most likely reason for the popularity of christianity

Which of the following is the most likely reason for the popularity of christianity in the roman empire?
unlike mainstream roman religion, in christianity a person's rank or status was not the measure of their spiritual value.
unlike mainstream roman religion, christianity accepted customs and religions from different places.
unlike mainstream roman religion, christianity forbid the practice of slavery.
unlike mainstream roman religion, christianity's monotheism offered an easy explanation of why bad things happen to good p