Try this. Go out and ask people who the top five most influential Christians were, and see how many people mention Constantine. Unless you’re exceedingly fortunate, probably no one will. A sad indication of how few people know any history of religion.
But without him, things would be a lot different, and he should be about #2 on the list. Paul, of course, is #1. (Jesus doesn’t count, he was a Jew.) Paul was the enabler. Without him, Christianity would probably have remained a small sect, an offshoot of Judaism. But after Paul hit his head and saw god, he went on a tear to bring everybody into the tent, and in particular, he did away with the scary Jewish parts of early Christianity. Don’t worry about dietary restrictions, he said: bacon-double cheeseburgers for everyone! Nervous about knives near your private parts? No problem, we won’t slice up your manly bits. You can join without being circumcised.
Freed from the tyranny of Leviticus, your average heathen was now free to experiment with this new cult. All the old ones (Isis, Apollo, Mithra, Zeus, Dionysus, Attis, etc.) were still around if it didn’t work out, but this new guy Jesus sounded interesting. And so, just like new religions in our era (Scientology, Mormonism), it began to gather a following.
As far as the Romans were concerned, however, the Christians were a bit of a nuisance. Unlike the cult of Apollo, who didn’t mind if you sacrificed to Zeus as well, the Christians kept the one key feature from their Jewish forbears, and insisted they were right and everyone else was wrong. This was a problem for some of the Roman emperors, who promoted the idea that they were gods, and should be worshipped as well. Plus you’d get the occasional group of rowdy Christians who would tear down a statue or temple devoted so some other god. So Christians weren’t entirely popular with the ruling classes.
But in 306, things began to change. Flavius Valerius Aurelius Constantinus Augustus was granted the title of Caesar. He was in York at the time, commanding an army against the Picts. As a general, he depended on the good will of the gods to be successful in battle. In 312, facing an army twice the size of his, he had a dream suggesting he adopt the sign of the cross of Christ as his standard in battle. (This dream didn’t come out of the blue, Constantine’s mother was a Christian and had probably been pushing him in that direction.)
After slaughtering the enemy troops and returning with his opponent’s severed head , Constantine decided that he’d stick with this Christ fellow, marched back to Rome, and gave his Ich bin ein Christian speech. This had an immediate effect. For one thing, if you believe your leader is a god, and he tells you there’s an even better one, you’re going take that seriously. (Imagine if Chairman Mao had suggested that China become a Christian/Muslim/Buddhist nation.)
Beyond that, however, Constantine’s conversion had another important effect. Being the head of government, Constantine brought the bureaucratic wheels of state into the picture. Before Constantine, Christianity was a disorganized hodge-podge. Basically, the beliefs in every region were slightly different. There were Arians, Donatists, Marcionists, Montanists, Pelgianists and all sorts of disparate opinions, with each group accusing the others of heresy. Constantine locked the bishops from the different regions in one room together at the First Council of Nicaea, and told them to get their shit together and come up with a consistant story.
Once that was done, the division between the accepted dogma of the church and heresy was essentially prosecuted by the state. Constantine eventually started the process of shutting down the temples of the pagan gods and raising taxes to build a brand new city full of Christian churches and architecture called (wait for it) Constantinople (now Istanbul).
When Constantine’s heirs came to power, they continued the process (with one short-lived exception) until in 380, Theodosius I made Christianity the state religion of the Roman empire, and it essentially became a crime to be anything other than Christian. While the church likes to pretend that it converted the west using itinerant missionaries and damn fine preaching, it was rather, at sword point or under force of law that it actually happened.