Rodney Stark, in his book Cities of God:
the Christianization of the [Roman] empire was not the result of ‘reactions to public calamity,’ but to religious influences per se. That is, religion did not merely offer psychological antidotes for the misery of life; it actually made life less miserable!
…The truly revolutionary aspect of Christianity lay in moral imperatives such as ‘Love one’s neighbor as oneself,’ ‘Do unto others as you would have them do unto you,’ ‘It is more blessed to give than to receive,’ and ‘When you did it to the least of my brethren, you did it unto me.’ These were not just slogans. Members did nurse the sick, even during epidemics; they did support orphans, widows, the elderly, and the poor; they did concern themselves with the lot of slaves. In short, Christians created ‘a miniature welfare state in an empire which for the most part lacked social services.’
It was these responses to the long-standing misery of life in antiquity, not the onset of worse conditions, that were the ‘material’ changes that inspired Christian growth. But these material benefits were entirely spiritual in origin.