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Christianity wasn't really the largest world religion, in either adherents or land area culturally dominated, until fairly recently.
For land area, the best way to visualize this is probably this YouTube video. Here's a screencap from the Early Middle ages:
What you should see there is that Christianity was fairly minor until it managed to covert most of Europe, which took until roughly the start of the late middle ages, and then it really took off after the "Age of Discovery" (early modern era).
For number of (perhaps nominal) devotees, the best thing to look at would be one of those new animated bar graphs. Unfortunately, I was only able to find one that goes back into the ancient era, and I'm not sure its trustworthy*. However, it at least does the job of showing that Christianity was an also-ran around the time of the fall of the Roman Empire, and didn't take the lead until at least the late middle ages.
* - It claims to have done "my own research" with no references, and actually misspelled "Christian" on the graph.
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Have you heard about Mythraism? It was a very close competitor of Christianity. So much so, Christianity copied from it. The halo behind saints was originally Mythraist, not Christian. Many Christian customs and rituals are remarkably close: the sacrificial meal and nativity story, for example. Mythraism is much older than Christianity, so it's far more likely Christians copied Mythras than the other way around.
The Isis religion was also very popular. That's where the Virgin Mary comes from.
Christianity wasn't immediately very popular from the outset, and certainly not in the entire empire. Christianity spread slowly through the Eastern Empire, and more slowly in the West. It took at least a full century before it became noticeable, and another century to gain prominence.
Contrast that with the spread of islam. Mohammed and his followers gained more adherents during his life and the following 50 years than Christianity in three centuries.