But those empires were constantly fighting one another, it took Christianized Rome for them to put aside their differences and build monuments that have lasted millennia.
The Church ensured that these wars were generally more of a competitive team sport rather than the actual wars of extermination that had occurred before, where generally one nation wiped the other out or subjugated and erased their identity. Theres a good reason why Europeans were suprised at the barbarity of WWI and WWII, because those were the first truly post-Christian wars the west had fought.