What aspect Christianity caused it to spread quickly?

Answer 1

it favored earthly riches because in the past religions you were not allowed to have riches you were to sacrifice them to your god but Jesus said you can have them because god will make you prosperous

Answer 2

Jesus told his disciples to go and spread the good news:

Mat 28:19 Go, then, to all peoples everywhere and make them my disciples: baptize them in the name of the Father, the Son, and the Holy Spirit,

Mat 28:20 and teach them to obey everything I have commanded you. And I will be with you always, to the end of the age."

The fact that Jesus disciples obeyed Him resulted in the rapid spread of Christianity. This was also helped by the fact that throughout most of the known world at that time there was a common language - Greek. Also the Romans as they conquered nations they built good roads to help them move their troops around so travel was relatively easy.