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Islam (to believe in the One and only Allah Almighty as God) was the religion of every prophet, therefore, no other religion is acceptable in the Court of Allah Almighty, however, now Islam refers to the religion brought by Prophet Muhammad Al-Muṣṭafā صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم; he was sent as a prophet for all people and as the last prophet. So if anyone claims to follow any other heavenly religion but does not believe in the Final Prophet صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم and his religion, then his claim shall not be accepted.