Would there be Catholics or any form of Christianity without this Guy right up there? His might has been felt all over the world. Crusades, Wars, gay rights, abortion, Religion as we have it today, etc. That is just the tip of the iceberg that sunk the Titanic. If it was not for Jesus making his religion more liberal, his religion would still be a horrific nightmare that we saw in the Old Testament.
This has been both good and bad. Bad because we have the Catholic church trying to keep the human race in the dark ages for centuries, but it is also good because we would have all been wild blood thirsty killers by now. No one would want to sacrifice lambs on a regular basis besides Isrealites and everyone else would go on to be Pegans. Eventually either worshipping evil enities or just becoming Athiests when Charles Darwin came along.
I guess second place would go to Karl Marx. China, Russia, North Korea, Cuba, etc. His ideas that were forged by Satan helped destroy the World. So in actuality Satan is the most influencial person in human history behind God, but they are not and were not men, even though in many Christian Religions Jesus is God. Even though he is the Son of god, and at one point, a Holy Spirit Zombie that walked among men.