In the 500s, the Roman church successfully evangelized Europe. They were united, although their flaw is that they did nor cooperate very well with the patriarchs of other regions. In the East, the church kept arguing over icons and theology that they could not get along.
Around that time, Arabian tribesmen congregated to Mecca and Medina for commerce. One of them, Mohammad, met some Christians and Jews while traveling Palestine. He later married a rich widow, and had time to meditate. He then began to preach monotheism, and taught against idolatry. These teachings were why the Eastern church began to argue over icons and graven images. They were pressured by Mohammad. He started in Mecca and then fled to Medina, and he converted followers to his faith.
These people of Islam, who believe in complete submission to Allah, eventually conquered Egypt, Jerusalem, and even Persia. While Christianity thrived in Europe, everybody else was threatened by the Muslims led by Mohammad. They soon built a center in Baghdad and became acquainted with the classic Greek writings of Aristotle. In this "Twelfth Century Renaissance," inventions, science, and literature thrived in the Arabic world. These were the intelligent people with the cool ideas. These were the Bill Gates and Steve Jobs. And like most intelligent people who have great ideas, they also want their ideas to conquer the world. And when this happens, the Church becomes intimidated and tries to either compromise with them or adjust their message to make it more palatable to the population, or they reexamine their beliefs and practices like they did with icons. Sometimes this works, but often they compromise so much that they lose their ground in the Word of God and in trusting the Holy Spirit and start to accept ideas that should not be accepted.
With Islam, all this happened, plus they wanted to conquer every corner of the known world. So, they lured the Church with their bright new ideas and then began to take advantage of them, and finally used force to conquer their lands. The Western tried to negotiate with them first with missionaries, then with the Crusades, but the Muslims would not budge.
It makes me wonder about missionaries today. There are so many brilliant people with great ideas and personalities who we want to impress for Christ. We want it so bad that we will do anything to get them in Church, even if it means using unbiblical means. They're tired of waiting on the Holy Spirit to do his work and try to get faster methods, and in this way they weaken their base in Scripture and the non-believers soon conquer them.
The good news is that there is still Europe. There is still a remnant Church. Our job is to reeducate our people on what we actually believe, the good, bad, and uncomfortable, and then resist non-believers while still loving them as people. You can love people without accepting their beliefs. Really, Christians should just witness to people by living in the world. Not with tracts or evange-cubes, but with just meeting people and living life. Perhaps it will spark a conversation that will lead someone to church. It's God that saves them anyway, not our schemes.