How did Christianity gain in popularity?

Answer 1

It was spread by Holy men and women of God as they were empowered by the Holy Spirit. The Holy Spirit made its entrance into Christianity on the day of Pentecost as promised by Jesus Christ. The promise of salvation of the soul thru repentance of sin was made possible thru the vicarious atoning death of Jesus The Christ. This promise of salvation and eternal life is thru the new birth is offered to 'who so ever believes on Christ who was resurrected after being crucified for bringing Christianity to the people. The coming of Jesus was in fulfillment of the Law and the prophets of the Biblical Old Testament.

Answer 2

Christianity gained popularity in the Roman Empire because of aggressive preaching by the followers of Jesus. The very early Christians were told to convert others as then they would get a higher place in heaven.

Answer 3

Christ's disciples told people about him after he had died. Originally, though, Jesus himself told people about God and Heaven and that belief in Jesus is the only way to heaven.