Jesus was not a Islam follower (the faith Islam was invented only 1400 years ago), so he could not be a Muslim. But  for Islam, Jesus is one of the four prophets; Mohamad being the last one; like Hindu people consider Lord Rama and Lord Krishna as incarnation of God. However, Christians and Muslims (followers of the religions) are more divided and fight/kill each other nowadays like Hindu people were divided between Baisnav and non-Baisnav in South India some 300-400 years ago and used to fight/kill each other.