Even though Christians and Islamic peoples have the same god (with different names), The biggest difference between Islam and Christianity is that Christianity dictates that Jesus Christ to be the savior of all Mankind, where as Islam does not.
There are also disputes over who was and was not a prophet, canonized scripture (the Koran as opposed to The Bible) and other beliefs.
I'd have to say Christianity for certain and with the rise of Islam, then Islam is number two.
Christianity, Islam, Judiasm
Islam and Christianity
Islam and Christianity.
Islam and christianity.
Christianity and Islam
Christianity Hinduism and Islam
Christianity and Islam
Judaism, Christianity and Islam
Judaism, Christianity, & Islam.
Christianity and Islam.
Hinduism, Islam, Sikhism, Christianity are main religions of India.