It was taught by Prophet Muhammad (peace and blessings of god be upon him).
I don't know the Most popular religion. But the most common ones are; Hinduism, Buddhism and Islamic.
Serfs are Islamic
The religion that Muhammad is favored is the Islamic religion. The Islamic religion regards Muhammad as its most important prophet, and pilgrims to sites he visited are important parts of the religion.
Iran
Islamic
The answer is in your Question: Islam and its followers are called Muslim
If people studied the Islamic religion in full, then no one would be attracted to such a religion.
"Islamic" means that it is related to the religion called "Islam". Such countries had, and still have, a population which, in their majority, was islamic.
Jerusalem. That is where the prophet Mohammed spoke and taught.
Shiites and Sunnis. Refer to question below
Islamic religion is the faith of Muslims who are spread all over the world.