Christendom, What happened to you?

Over the course of 1500 years, from the death of Jesus to the era of exploration and Martin Luther, the basic values of Christendom changed tremendously. I believe that Jesus was just a man who preached "be good people", which was absolutely a righteous cause. But I believe that human's are also selfish, and when Christianity rose as the major religious power in Europe, those selfish humans saw the ability to bend it to their will to accomplish personal goals. For instance, European monarchs would use the "because god said I was king" excuse to uphold their power. And because there was no way to prove or disprove this, the peasants believed it. This allowed monarchs to raise money to build armies to conquer. This is where Luther had the same problem as me, and so he attempted to bring Christianity back to the more traditional message of "be good people". Of course, this problem is still existant today by politicians and heads of churches, using the bible and Jesus to prevent the development of society as a whole for their own selfish purposes.