And of course the Western Empire didn't go down the tubes till AFTER it became Christian (Gibbon asserts it was in part BECAUSE it went Christian, but Decline and Fall of the Roman Empire is one of those books everyone has heard of but few have actually read).
Always cracks me up when Social Cons bring up the decline of Rome when they're riding a hobby horse about some moral issue, unaware that it was when Rome was pagan and decadent it was growing in power, and the collapse was after Christianity.
Gibbon also debunks a lot of exaggeration of the persecution of Christians under pagan Rome as well, and notes most Christians were killed by other Christians during that time.
Why is the fall of Rome a bad thing? Big Government, High taxation socialist government based on an economy of commune farms worked by slave labor.