In a below paragraph, Allah (C.C) [God or Lord] says lots of community, prophets and scripture a muslim which is before the prophet Muhammed to the verses from the Koran. The root meaning of the muslim is ISLAM.
As a result, Allah (C.C) [God or Lord] says MUSLIM to Ibraahim (Abraham), 'Iesa (Jesus), Moosa (Moses), Ya'qoob (Jacob), Ismaa'il (Ishmael), Ishaaque (Isaac), Yusuf, Sulaimaan (Solomon), and so their religion is ISLAM. In that manner, in the same religion different Shari’as recognize and start to dialogue between themselves.
1.Our Lord, and make us Muslims [in submission] to You and from our descendants a Muslim nation [in submission] to You. And show us our rites [of úajj and Ôumrah] and accept our repentance. Indeed, You are the Accepting of repentance, the Merciful.
(Surah al-Baqarah, 128)