Paganism, and hellenism would likely have evolved too in acceptable religions, just different than ours. In a few thousand years, we might ask ourselves: "Was the death of Christianity a bad thing or a good thing?", and there is no true answer to that. I think you can be spiritual and religious, but moderately.
{Citation desperately needed!}
The idea that there are natural inevitable currents of history/politics/religion/whatever is one of the silliest ideas in the social sciences. It takes the accurate premise that certain changes require some necessary precursors to happen and makes the illogical leap that having those precursors happen is sufficient to guarantee the change will happen.
For that matter, absent Christianity, likely some other mystery religion would have supplanted traditional Greco-Roman beliefs within the Empire, of which only some were based upon those old beliefs.