In this scenario, Constantine the Great of the Roman Empire was never born, thereby never converting the Roman Empire to Christianity.
The Fall of the Roman Empire
In this scenario, the Roman Empire would still have fallen, but much earlier than in our timeline. It fell into civil war in 306 A.D, and after that, it was divided into the Eastern and Western Empire. Over the next century, these two empires were slowly gobbled up by the barbarians up north.