Check out this History Channel documentary about how the Roman Emperor Constantine used the idea behind the pagan cult of Mithra to create the modern version of Christianity. The investigation team goes to Rome and uncover compelling archaeological and architectural evidence to support this. The similarities and parallels between Mithra and Jesus are striking and uncanny.
I guess the logical conclusion here is that since the Romans were unable to destroy Christianity, they decided to infiltrate it and use it for it's own purposes of political control instead. So they created a version of it that would suit their interests.
Also check out this long 3 hour documentary on the first 1,000 years of Christianity. It's very interesting and covers a lot of history.
Here is the sequel documentary on the second 1,000 years of Christianity. It's also nearly 3 hours long.