Ancient Rome and the Roman Empire were militaristic societies. But the Roman military campaigns were not motivated by religious considerations. The Roman pagan religion was inclusive, and tolerant of local Gods and cults. The Romans would often incorporate the local Gods into the pantheon of their pagan Gods. The number of Roman Gods kept increasing as they kept conquering lands in Europe, Asia, and North Africa. Every new conquest brought new local Gods to the Romans.
It was after the rise of the monotheistic faiths, which did not allow belief in any God except the one true God, that the wars which were supposedly for the defense of the faith became common. The wars of the monotheistic cultures were religious in name only. To a large extent these wars were grounded on geopolitical considerations—the ambition for capturing land, wealth, and slaves; for taking control of strategic trade routes and sea lanes; and for settling scores and attaining glory.
In my readings of history, I have not discovered a single war that was fought for purely religious considerations.