Piety: the Jewish Messiah cult was the basis for Christianity that became a powerful competitor with the emperor cult. It is not mere chance that both the emperor cult and
early Christianity flourished in the same area, Asia Minor. You could even say that Christianity was the better emperor cult, because Christianity was more easily able to integrate the Eastern concept of individual piety, passed down by Judaism and other religious movements.
The emperor cult remained a state cult that had little to offer in terms of individual piety. That is why Christianity was able to become the state religion of Rome under Emperor Constantine and his successors.