Christianity and Islam share much more as religions than most people realize. The two religions are tightly interwoven because of the shared ancestry, not only geographically but historically.
Both religions are based on the same monotheistic principle of one supreme entity. Christians refer to God and Muslims to Allah. Many of the important figures for each religion are also shared, but their roles are vastly different.
Christians believe Abraham was a favored man of God who had many sons and whose lineage included Jesus of Nazareth, who is also called Christ. Jesus is believed to be the direct son of God through the immaculate conception of a virgin, Mary. Jesus became a preacher and died on the cross at the hands of the Romans to be resurrected three days later to ascend to Heaven, the place of God. They believe Jesus will return in a final judgment of man.
Muslims also believe Abraham was a favored man of Allah who had many sons. There is some dispute about whether Jesus was from the lineage or a direct descendent of Allah, but Jesus is viewed as an important prophet whose message was misunderstood. They believe Jesus ascended to Paradise, the place of Allah, and will return as a prophet along with Muhammad in the final judgment.
Each religion is also divided into different sects. Christians have Catholics and Protestants as the two main divisions with each having smaller subsets such as Baptist, Methodist and Mormons. Islam has three major sects; Sunni, Shi'a and Kharijites.
Many customs are also shared. Prayer is mandated in each. Fasting is another shared tradition. Muslims do not eat during the day during the month of Ramadan. Christians have a similar time of fasting during Lent. Thoughts on modesty and the role of women are comparable, although traditionally Islam takes the view to a more extreme manner by forcing women to be completely covered.
One major difference is the manner of achieving Heaven or Paradise. Christians believe Jesus is the son of God and one must ask forgiveness, confess sins and adhere to the teachings of the Bible to enter Heaven. Muslims believe only those who pray to Allah, acknowledge Him as the only god, and follow the Qur'an will reach Paradise.
Another major difference is the manner in which Christianity and Islam view conversion. Christians believe in missionary work, bringing the teachings of the Bible to everyone; conversion must only be done by the true confession of the individual. Islam has a more heavy handed view of conversion and believe it can be done via force and Allah demanded conversion by force of those deemed "infidels."
Although the world has currently a major divide among Christians and Muslims, their religions share more than they differ.