Mohammad is probably up in Heaven with Moses and Jesus, but the only difference is that Jesus rules over Heaven at the Right Hand of God the Eternal Father (who Muslims call "Allah", Jews call "Yaheweh" and some Christians call "Jehovah", "Abba", or "Father".) Jesus also stated "I Am" to the Jewish elders; just as God the Father stated "I Am" to Moses. Jesus rose into Heaven both Body and Spirit after rising from the dead; an event that no historian could disprove, but rather all proponents point to this event as historical fact. Moses was not "The Way, The Truth, and The Light" as Jesus was, and Mohammad is was on Moses' level as a prophet. Jesus stated here that He is The Way, The Truth, and The Light of God. Moses and Mohommad were His prophets. Just as a man can be father, son and husband to his daughter, mother, and wife (repectively), so did God make a complete relationship with the Human Race through Mary, the Mother of Jesus, by relating to her as Father, Son, and Spouse (God the Father, God the Son, and God the Holy Spirit respectively), hence the triune God-head, three persons in One God, or probably better worded as three aspects of One God.
Anybody who is a true follower of Christ and his teachings, believing in Christ as the only Begotten Son of God the Father, is a true Christian; regardless of whether they are Catholic, Orthodox, Anglican, or other Protestants. These are not separate religions, but rather different sects of the faith; just as in Islam, Buddhism and Hinduism you have separate sects [en.wikipedia.org
Catholism is the largest sect, being the western half of the original united Christian Church (Orthodox being the eastern half and the second largest sect). Both Orthodox and Catholism are very similar in all core beliefs; the only differences being in the traditions of certain celebrations, such as how to perform a wedding, the length of the Christmas Season, etc. Yet the core religious doctrine of Catholics and Orthodox are very much the same. This is because, if you put both Catholic and Orthodox Churches together, you get the original Apostolic Church, founded by St. Paul and St. Peter, based on the teachings of Jesus Christ. Protestants (including Anglicans) are those who Protested against Mother Church (hence the name Protestant) due to conflicts of political and religious beliefs at the time (mostly political). These were the ones who broke away from the Church in order to found their own version of Christianity. To this day, the Catholic Church is known to continue its eccumenical efforts to reunite the entire Christian faith under one doctrine through proper communication.
Now whether Christianity is larger, or Islam is larger, is hard to say. It is a well known fact that Christanity is far more wide-spread than any other religion in the world. Islam is probably the most visible amoung orthodox, tradiitonal muslims, as you can spot them very easily by their manner of dress. For most Christians who are not clergy, you would have to check their pockets or under their shirts for any physical symbol of faith. Just as thousands of people convert to Isalm each day, thosands of other people convert to Christanity each day as well. There is also Hinduism, Buddhism, Jewdaism, and other major religions who receive a multitude of converts on a daily basis at any given time.
However, it is known that the two largest religions are Christianity and Islam, though most unbiased, third-party sources would demonstrate that Christianity is the larger of the two. If you want to know for sure which one is bigger, just Google it.
Here is a link to what I found.www.adherents.com