Rome was polytheistic and noted as one of the greatest empires of all time because of their unity as a "roman", they were proud of the state, not a specific religion(there's your nationalism/patriotism), not over a god. They expanded for tangible goods and land, not to impose a religion of "one true god". In fact one of the attributed down falls of the Roman empire was the adoption of Christianity, which literally split the empire in two.(but that is altogether a different story, but shows the impact of religion on a culture).
This is the narrative that Gibson suggested, and it's laughably flawed by today's standards.
Entire armies would refuse to do battle until the Sacred Chickens had been consulted and reported back favorably. (I'm not making this up, a priest would feed chickens a corn cake, and if the chickens ate it, the gods were pleased). If grown men would refuse to do battle on what a chicken said to them, I find that as religious or more religious than most examples of Christianity. (Again, let me repeat, they would ignore state and civil duties because a chicken refused to eat cake)
The Eastern and Western Empires were two different beasts, and that conclusion is evidenced by how the Romans themselves split it between two Emperors. Gibson's thesis conveniently ignores that the Eastern Empire lasted until the late 1400's, and was equally Christian as the Western Empire. Again, Gibson portrays the Eastern Empire as a thousand years of decline, but come on. There's no decline that lasts a thousand years.