It blows my mind that two of the biggest moments for Western world culture happened in a single generation. Is there anything that may account for this?
Can you clarify which years and events you're referring to? The Fall of the Roman Republic is generally referred to as the period of 2nd and 1st century BC, and the first inklings of the Christian Church were in the 1st century AD - 100-300 years difference.
However, if you're talking about 395 AD vs. 476 AD, that is still 80 years difference. Please note that it was only the Western Roman Empire that collapsed in 476, the Eastern Roman Empire, also Christian, survived for many centuries to come.