While some of the answers here are certainly interesting, it’s a little difficult, to say the least, to provide any sort of answer to such a broad counterfactual question. I mean, it’s difficult enough to answer counterfactuals when they apply to single figures in short time spaces, e.g. what if JFK had not been assassinated.
But you’re essentially asking:
What would have happened if probably the single most important social, political, intellectual and religious institution (for want of a better word) in Europe over the course of two thousand years had simply not existed?
Answering such a question cannot be done simply by removing Christianity and then pretending everything else would have progressed in a similar way. Removing such a massively important piece of the historical picture would have brought about changes, small and large, that we can barely even conceive from our present-centred position.
I’m not trying to stifle debate here - by all means, go on hypothesizing - but be aware that it is NOT simply a case of:
Europe - Christianity = Islam, or something similar.