This was the religion that united Europe together during the dark ages

The liberal European Union today has also united Europe. What is important is on what terms this unity is achieved.

when it faced an existential threat from the Islamic caliphate

It is very difficult to imagine Islam existing without Christianity.

Contrarywise, we must ask what the repercussions were from the Wests rejection of Christianity.

A lot of people are making this argument, but I think it would be a very good question to ask if the social situation in the West today is a result of rejecting Christianity specifically or of rejecting religion more broadly. It is also a good to ask why this rejection occurred in the first place.