All three of the monotheistic Abrahamic religions of the Book have fought holy wars. ISIS is now fighting a holy war in the Middle East because it is the will of Allah that they kill infidels. The 31st Chapter of Numbers states that Yahweh (the god of the ancient Israelites) commanded them to slaughter the Midianites (men, women, children, even the livestock) because the Midianites did not worship Yahweh. Pope Urban II told the Crusaders it was God's will that they march to the Holy Land and kill infidel Muslims. We would all be better off if everybody converted to Buddhism because Buddhists don't have gods who tell them to kill infidels.
The Roman Empire was not godless. The Romans were pagans until the Emperor Constantine converted to Christianity. The pagan (they were polytheistic) Romans did not expect conquered people to convert to paganism (they were tolerant except when people rebelled against Caesar; then they crucified people).
Once Christianity became the state religion of the Roman Empirre, the Christian Romans started persecuting both pagans and Jews. European anti-semitism can be traced all the way back to the Christian Roman Empire.