There is a lot of misinformation here.
Islam spread by the sword?
Christians especially were allowed for the most part to keep their religion, they converted willfully if they did become Muslim. Also note, that Indonesia the most populated Muslim country in the world was not even taken by Islamic conquest. And today, Islam is the fastest growing religion in the U.S. and the world. Not by sword, but by free will.
Your knowledge of Islam is quite laughable. Read the Qur'an. Jesus is a MAJOR prophet in the Qur'an. There is a large chapter on the Virgin Mary as well. There are countless verses on the life of Jesus and his teachings. There was nothing gradual of adding Jesus. Don't make things up. From the beginning of the message of Muhammad, Jesus and all the other Israelite Prophets were a major part of revelation, because those Prophets brought the same message. Muhammad was culmination of all that.
Just to add to this point because many Christians don't realize what Jesus is to Muslims. He is one of the 5 major Prophets who had revelation, he is the Messiah who will return before the judgment day, he performed many miracles, he was sinless, he foretold the coming of Muhammad, he spoke in the cradle to defend his pure mother, among many other things. Don't act like these things were added gradually, that is just a false claim. They were in the beliefs in the time of the Prophet, before any Islamic expansion. They are in the Qur'an which was finished in revelation just before the death of Muhammad.
The Qur'an was revealed over a 23 year period through Muhammad. That Revelation has been intact, in the same exact language, with no changes, alterations, deletions, or interpolations for 1400 years since the death of Muhammad. It didn't spring down instantaneously, but it wasn't altered after it was complete and the timing is clear.
Islam co-opted Christianity? You know very little about Islam. If you read the Qur'an and other Islamic texts you will realize its teachings came to correct deviated beliefs of those who came after Christ and came to bring back the true teachings of Christ, this is what they say. The Qur'an testifies to the original teachings of Jesus, the Injeel, loosely known as the Gospel. How could it take things from Christianity when it gives its own unique insight into the original teachings of Jesus, in many ways, contradicting present-day Christian theology.
As others have pointed out, in Islam Moses, Jesus, and Muhammad brought the same divine religion at different stages in human history. According to Islamic doctrine, there is no contradiction between these different messengers. They merely came at different times, to a different context, and to different people, but the same message.
The followers of Christ in the early years after Jesus did not believe in those things that you told me. They were as much if not better followers of his teachings than the majority of Christians today. They were closer to Jesus and his teachings.
If you're gonna make claims about Islam, I advise you to actually study it yourself in depth from primary sources.