The Roman Empire collapsed when the christers took over.
Invasions by Barbarian tribes. The most straightforward theory for Western
Rome's collapse pins the
fall on a string of military losses sustained against outside forces.
Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the
Empire's borders.
The 18th-century historian Edward Gibbon was the most famous proponent of this theory, regarding Christianity having contributed to the fall of The Roman Empire...... but his take has since been widely criticized.
While the spread of Christianity may have played a small role in curbing Roman civic virtue, most scholars now argue that its influence paled in comparison to military, economic and administrative factors.