In hoc signo vinces!

Rome under Religio Romana: *Becomes the greatest Empire in history and reaches its peak under Jupiter*

Rome under Christianity: *Declines and is destroyed by Germanic pagans and The Ottomans.

The Eastern Romans routed the by then Arian Goths and Vandals, could only be defeated by the Muslim caliphs at their very weakest after fighting Persia to an absolute standstill, and fought the Turks for centuries before Constantinople finally fell.
Roman-descended peoples such as the French, Spanish, Italians, and Portuguese conquered far more people and territory under Christianity than under paganism anyway.

Conquered more empty land and could not hold said land. Not much of an accomplishment. Pitiful compared to 1st and 2nd century Romes near-invincibility. A far cry from losing the entire Empire now isn't it.

Rome under the Gods guidance became an empire that mattered, and abandoning the Gods caused Rome's downfall.

Italia and Africa under the Goths and Vandals weren't exactly empty but, regardless, the Exarchate of Africa remained firmly under Byzantine rule until the Islamic invasions, and even the Exarchate of Ravenna remained in Eastern Roman hands for over a century and a half. Given the Eastern Romans' restored domination of the Mediterranean, I'd certainly say that they were an empire that mattered.

My point exactly. Christianity was defeated by Islam back then just as Islam is defeating Christianity today.

Pagans, specifically Hellenists where not defeated. Instead they converted to christianity and lost.

Paganism should've offered a better and more convincing religion that Christianity then, otherwise they wouldn't have all converted. If you believe in "the law of the victor" instead of objective morality, then surely you would agree with that? Christianity has undoubtedly been victorious over paganism because all the pagans converted to it. The remarkable success of Christianity's spread and its ascension into the predominant religion of Western civilisation has been like no other.