^I don't think that's quite what he said. IIRC it was more along the lines of "if it weren't for the Dark Ages we'd be colonising space by now". What happened in the Dark Ages was a massive technological regression across Europe caused (by and large) by the collapse of the Roman Empire and the final death throws of Byzantium.
While it may have been a contributing factor in a social context, the spread of Christianity in the west and Islam in the middle east and north Africa was more a symptom of that collapse than a direct cause. They were basically filling a void left by the old religions. Indeed, it was thank to the Islamic nations that a lot of Greek and Roman knowledge was preserved while we were busy burning witches and playing musical thrones (think musical chairs, but with more beheadings and less music.)