What factors allowed Christianity to spread within the Roman Empire?

Several factors allowed Christianity to spread within the
Roman Empire.

The main cause of its spread was that it was an
evangelical religion, sending missionaries across the Roman Empire. The decision, made
first by Peter and then expanded upon by Paul, to preach to gentiles as well as Jews and
not to require gentile converts to follow Jewish law increased its
appeal.

The Pax Romana, as well as extensive road and shipping network
of the Roman Empire, facilitated the physical circulation of missionaries and letters,
and thus was another enabling condition for the spread of
Christianity.

Finally, polytheism was a system in which new gods were
normally introduced freely and their worship allowed. Where Christianity, as Judaism
before it, ran into trouble was not in worshipping a new God, but in insisting that this
was the only God and refusing to worship the state gods and deified emperor, an act that
was viewed as a form of treason, until Constantine who legalized
Christianity.