Question

what are the reason christianity grew?

Answer 1

in rome?

Answer 2

If your talking about christianity growing in rome,

First, the good news that God in Christ is reconciling the world to Himself, verified by signs and wonders done in the name of Jesus.

Second, the sense of community and oneness (even to the limited degree it manifested) in the assembly was attractive, particularly to the downtrodden and outcasts of society, many of them slaves.

Third, the Roman Road and the Pax Romanus, not to mention the commonality of the Greek language provided practical logistical advantages which had not been available at any previous time in history. (God timed that just right, in other words.)