Before I answer that, let's play a game. Let's invent a religion that you want to spread and 'go viral.' What could we do to make a religion that is immensely popular?
I would suggest that the following steps might be advisable:
- Understand your motivation: why are you inventing the religion? Do you want fame, power, money, sex? Make sure to include opportunities to get these or it was pointless to go through all this trouble.
- Base it on private revelation -- you don't want to say things that a reporter could investigate and prove to be false.
- Define your deity/philosophy in terms that are popular in the culture. It is important that people want to follow the religion.
- If you can't achieve #3 because your deity or philosophy is too weird or unpopular, then keep it mysterious: let only the priests know this truth, while having a "softer" face for the masses.
- Have a popular, well-spoken, or otherwise powerful "face" to serve as a prophet to the masses.
- Give people achievable steps or checklists so that they know they are growing in their faith.
Sound reasonable? I would argue that such a religion would catch on very well...because that is a formula which has been used by most religions throughout history. Consider the above 6 points in the following world religions:
Islam, Mormonism, Scientology, Christian Science, and Jehovah's Witness all adhere to at least 5 of the 6 steps above, with three of them adhering to all 6. It just makes sense: these are things which would be common-place considerations if one wanted to come up with a new religion.
And yet...Christianity doesn't have those. Not any of them. It simply does not bear the hallmarks of a made-up religion. Consider each point:
Christianity's leaders didn't get earthly gains
The leaders of Christianity did not find themselves wealthy, nor did anything they do give a likely chance of this happening. Jesus was crucified by His mid-thirties, and eleven of His twelve apostles were martyred (with number twelve, John, dying in a prison of old age). The religion suffered persecution after persecution for years, yet spread, rather than dying. As Tertullian famously said, it was the opposite--the blood of martyrs seemed to be a seed that spread Christianity rather than slowing it down.
Christianity did not claim a private revelation, but a very public one
Jesus didn't hide. Unlike all of the religions above, Jesus' miracles and teachings were witnessed (or not witnessed) by thousands. If the apostles were making it up, people could literally go check and find out if they were lying. Indeed, the Gospel of Luke is basically that: Luke's patron, Theophilus, hears of Jesus and commissions Luke to do a little investigative journalism and see what the witnesses tell him.
Christianity is the only major religion which made claims that people who were living could go and verify. When John's Gospel came out in 90-100 AD, you could walk to Cana and see if anyone remembered the wedding where Jesus turned water into wine. When the synoptic Gospels were circulating in 50-60 AD, you could walk to Galilee, or Capernaum, or Nazareth, or Jerusalem, or Bethlehem, and find corroborating witnesses to back up the story. And, if these things didn't happen, it would have been easily disprovable. Interestingly, no one claimed Jesus' miracles were lies. The Jews argued that His power came from Satan, but no one tried to say that they didn't happen. Why? Too many witnesses!
If you are making up a religion, you do not do this! If you are making up a religion, you do not say, "In 1999, while George W Bush was Governor of Texas, a man came to downtown Houston and fed 5,000 people using only a basket of food." Journalists would just go to Houston and try and find evidence, photos, or witnesses.
The fact that the Christians were so careful to give historical milemarkers, and that they gave so many, and that the Jews and Romans who opposed them never called any of these claims false, is very convincing evidence that the events did, in fact, happen.
Christianity did not preach a popular God
In a culture of honor and shame, Jesus is not the kind of God you would make up. First, you wouldn't make up an exclusivist Jewish God--the Jews were already looked down on by the Romans as atheists, since they denied the existence of the Roman pantheon of pagan gods. Second, you wouldn't choose a man who was executed for sedition--that was viewed basically the same as terrorism today.
Try and imagine inventing a religion today, in America, that is based upon claiming that an al-Qaeda terrorist who was executed by the American government was actually the one true god incarnate. You won't get very far. Yet that is a good analogy for the Christian movement: again, this is not something anyone would have made up.
If the deity is weird, hide it
Jesus as a deity would have been very weird to Romans. To claim that someone other than an emperor was man-God, much less a dead Jewish seditionist, would have been laughable. Add to that the beliefs that Romans saw as "strange superstitions"--the strict sexual laws, the weird Sunday worship services in which the 'body and blood' were consumed, the refusal to serve in public office or take oaths to Rome--and you can imagine that the Romans would see Christians as very odd.
Yet Christians did not hide these things; they did not withdraw from the world. They professed publicly, even to the point of death if needed.
Have a powerful leader or prophet as a face
Again, there were no early powerful adherents of the faith. Joseph of Arimathea, as one member of the Sanhedrin, was the most well-known member from the Gospels, and he keeps pretty quiet. Ditto for Luke's patron, who goes by the name Theophilus instead of his real name (perhaps implying that he is involved in Roman government in some capacity).
No, this religion was started by mostly-illiterate fishermen and farmers--not exactly the elites you want to be the face of your new religion.
Achievable steps and checklists
Christianity is again different from the above because it gives you no list of "do's" which will get you to heaven, or keep chaos at bay: quite the opposite. It declares flatly that no one is capable of attaining God's favor, and yet He gives it anyway. This is the "Good News" which many find, frankly, repugnant.
So why is Christianity different from every other faith? Because it's not the kind of thing we would have made up: in every way, this is a faith which made no sense in the culture where it blossomed. It should have died if false...it didn't.