Christianisation was one of the divisive factors in the late Roman empire in fact, not a unifying element. The pagan Roman empire wsn’t very bothered about what people believed or said they believed; Christians spent their time arguing with each other about Homoousianism and/or Homoianism and/or Homoiousianism and persecuting people with the wrong opinion.
One reason for the swift muslim conquest of much of the empire was that persecuted “heretics” weren’t too eager to fight for their persecutors.