That is a very good Q.
"Christians"Never call themselves that in first generations.Same goes for Jews.
As for Buddhists, it is a different story. Their founder is a man not a good. they did not received Revelation from Above.
Muhammed (Peace be up on him) was not founder of Islam. He reserved Revelation from God Al Mighty.
Same goes for Jesus, Abraham, Moses, and many others.Islam is from Adam to last Messenger Muhammed. They were all Muslims.
When you read about Islam you will know why>
All Muslims agree (or let me say 99.9% of them) that a Muslim is the one who
Witnesses that there is no god but one (named Allah) and that Mohammed is the last messenger of Allah.
Prays five times a day (as specified in the prayers time table)
Fasts one month a year (namely the 9th month of the lunar calendar, known as the month of Ramadan).
Pays 2.5% of his annual savings to the poor and the needy (called Zakat)
Performs pilgrimage to Mecca (called Hajj) at least once in his life provided he is capable of doing that.
These are called the five pillars of Islam. The first pillar is a necessary one to be a Muslim because every thing else is based on it, but the other four are reflections of this declaration
By performing the five pillars, you pass the test of your life and enter paradise (Al-janna) after death. But to be an excellent Muslim you have to do more than that. Here are ways to get extra points and make you closer to Allah.