All the prophets including Noah, Abraham, Moses, Jesus and Mohammed invited people to worship God alone without associate as in the First Commandment which is more important than all the other commandments.
Second, the faith in God is not completed by the faith in Jesus or any other one; but Jesus himself and all the others invited to God's worship.
Therefore, the follower of each one of these prophets in the time of that prophet; such follower is rightly guided, and God is well-pleased with him, if he works the righteous work and gives the charity in addition to his belief in God and in the prophet of his time.
While refusing to believe in the next coming prophet indicates the sticking to traditions and the rejection of God; because why does he believe in the previous prophets and refuse to believe in the newest one?
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Muslims, in addition to peoples of all other religions, will be put to examination: why don't they believe in the Mahdi, while he invites them to God alone without associate, and in addition gives them the correct interpretation of the Quran?
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[Some Jews converted and became Muslims, but they forbade the camel meat for themselves, according to their custom of the Judaism; therefore, concerning them, God revealed this aya:]
Quran 2: 209. O believers! Comply and submit yourselves totally to all the commands of God a, and follow not the footsteps of Satan b; he is an obvious enemy c to you.
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a therefore, don't practice some commands and abstain from others.
b i.e. don't follow the suggestions of Satan and what he designs for you to prevent you from eating the camel meat.
c i.e. his enmity to you is so obvious.
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[A group of Jews converted and became Muslims: they were Abdullah son of Salam, Asad and Usaid sons of Ka'ab, Tha'alaba son of Qays, the nephew of Abdullah son of Salam (son of his sister) and Benjamin;
they said: "Mohammed, we believe in you and your book and in Moses and the Torah, but we don't believe in Jesus and the Gospel; therefore, this aya was revealed:]
Quran 4: 136. Believers [in Mohammed and the Quran], believe in God and His messenger [Jesus son of Mary] and the scripture [: the Gospel] which He revealed to His messenger [Jesus], and the scripture [: the Torah] which He had revealed before.
And whoso disbelieves in God and His angels and His scriptures and His messengers and the Last Day a, has surely gone far [from the truth], far astray [away from the correct way.]
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a i.e. the sending to the Next Life.
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About the same group of Jews who believed in Mohammed but did not believe in Jesus, the following aya was revealed:
[A group of Jews believed and said: “We believe in you, Mohammed, but we do not believe in Jesus, for he was a magician!”
Therefore, this aya was revealed in reply to them:]
Quran 57: 28. O you who believe [in Mohammed], ward off God ['s punishment, concerning your denying Jesus and your claim that Jesus was a magician], and believe in His messenger [Jesus, the son of Mary.]
And then [if you believe in Jesus], [God] will give you two portions of His mercy [: in the world of souls, and on the Doomsday a, and appoint for you a light wherewith you walk among souls] and forgive you [your past sins;]
for God is Most Forgiving [to those who repent], Most Merciful [to those who regret.]
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a That is according to your belief in Jesus, and your belief in Mohammed.
All the interpretations is by the late interpreter Mohammed-Ali Hassan Al-Hilly.