The Quran instructs both Jews and Christians to follow the guidance given to them in the Torah/Law and the Injil/Gospel:
But how do they come to you for decision while they have the Taurat (Torah), in which IS the (plain) Decision of Allah; yet even after that, they turn away. For they are not (really) believers. S. 5:43 Hilali-Khan
What makes this text interesting is that it discourages the Jews (and presumably the Christians) from turning to Muhammad for guidance since they have the Torah which contains the plain decision of Allah!
And We caused Jesus, son of Mary, to follow in their footsteps, confirming that which was (revealed) before him in the Torah, and We bestowed on him the Gospel wherein IS guidance and a light, confirming that which was (revealed) before it in the Torah - a guidance and an admonition unto those who ward off (evil). Let the People of the Gospel judge by that which Allah hath revealed therein. Whoso judgeth not by that which Allah hath revealed: such are evil-livers. S. 5:46-47 Pickthall
Here Christians are told to consult the Gospel since it is guidance and light. Hence, the foregoing verses are basically telling the Jews and Christians that they do not need Muhammads Quran since they have all that they need in the Law and the Gospel.
The Quran goes on to tell Muhammad to judge by the revelation given to him:
And We have revealed to you the Book with the truth, verifying what IS before it of the Book and a guardian over it, therefore judge between them by what Allah has revealed, and do not follow their low desires (to turn away) from the truth that has come to you; for every one of you did We appoint a law and a way, and if Allah had pleased He would have made you (all) a single people, but that He might try you in what He gave you, therefore strive with one another to hasten to virtuous deeds; to Allah is your return, of all (of you), so He will let you know that in which you differed; And that you should judge between them by what Allah has revealed, and do not follow their low desires, and be cautious of them, lest they seduce you from part of what Allah has revealed to you; but if they turn back, then know that Allah desires to afflict them on account of some of their faults; and most surely many of the people are transgressors. S. 5:48-49 Shakir
The above references explicitly say that Allah appointed for every one a specific law (shariah) and way, since his purpose wasnt that all the people should be united as one. This implies that each group received laws that contained major differences between them, that none of the prescribed ways that Allah gave are identical with one another, and that this is one of the reasons why the nations became divided. In the words of Ibn Kathir:
Allah has sent down various laws to all previous Messengers, but they all agreed upon the same creed (Tawheed) All the former religions of the previous Prophets have different laws with regard to orders and interdictions. One thing could be lawful in a religion but unlawful in another, vice versa. And a law could be lenient in one but harsh in another. However, the last Shariah sent down to the seal of all Prophets, Muhammad, has abrogated all the former religions and laws [sic], and Allah has prescribed it to mankind and Jinns, to Arabs and non-Arabs.
Allahs verse, <Had Allah willed He would have made you one nation, but that He may try you in what He has given you > means that He has ordained different laws in order to test His servants accordingly, and to reward them or punish them according to their deeds (Tafsir Ibn Kathir: Surat An-Nisa, ayat 148 to 176, Surah Al-Maidah, ayat 1 to 81, abridged by Sheikh Muhammad Nasib Ar-Rafai [Al-Firdous Ltd., London, 2000: First Edition], Part 6, p. 182)
Ibn Kathir clearly saw the problem in what the Quran states and tried to somehow resolve the dilemma. The references of Sura 5:43 and 47 do not say that these specific injunctions to the Jews and Christians to follow their own Scriptures are for the period of time before Muhammads advent and that Muhammads Quran has now come to abolish/abrogate these other laws. On the contrary, the verses expressly exhort the Jews and Christians to abide by the Torah and the Gospel, despite the fact that the Quran has already arrived.
Note, once again, what Sura 5:43 actually says:
But how do THEY come to YOU for decision while THEY have the Taurat (Torah), in which is the (plain) Decision of Allah; yet even after that, they turn away. For they are not (really) believers. Hilali-Khan
Notice the reference to the Jews coming to Muhammad, i.e "how do THEY come to YOU," showing clearly that this is referring to an event which took place at Muhammad's time. Thus, this injunction to the Jews that they should consult the Torah instead of turning to Muhammad cannot be referring to the time before the Quran was composed. Clearly, Ibn Kathir is wrong. He tried to resolve this problem by claiming that this competition is only for the past, but now it is only the Quran that is to be followed. But that is not what the Quran says.
This now brings us to a dilemma resulting from what the Quran says elsewhere, one which Ibn Kathir clearly saw and tried to reconcile. Muhammad is told to judge the Jews and Christians by the revelation given in the Quran, and that the Quran confirms the previous Scripture, namely the Torah and Injil. The Quran even commands Jews and Christians to follow the Quran:
O children of Israel! call to mind My favor which I bestowed on you and be faithful to (your) covenant with Me, I will fulfill (My) covenant with you; and of Me, Me alone, should you be afraid. And believe in what I have revealed, VERIFYING THAT WHICH IS WITH YOU, and be not the first to deny it, neither take a mean price in exchange for My communications; and Me, Me alone should you fear. S. 2:40-41 Shakir
And when there came to them a Book from Allah VERIFYING THAT WHICH THEY HAVE, and aforetime they used to pray for victory against those who disbelieve, but when there came to them (Prophet) that which they did not recognize, they disbelieved in him; so Allah's curse is on the unbelievers. S. 2:89 Shakir
These next two texts enjoin the Jews and Christians to do what is commanded in Gods revelations, i.e. the Torah, the Injil etc.,
Had they PERFORMED the Torah and the Gospel, and what was sent down to them from their Lord, they would have eaten both what was above them, and what was beneath their feet. Some of them are a just nation; but many of them -- evil are the things they do. Say: ‘People of the Book, you do not stand on anything, until you PERFORMED the Torah and the Gospel, and what was sent down to you from your Lord.’ And what has been sent down to thee from thy Lord will surely increase many of them in insolence and unbelief; so grieve not for the people of the unbelievers. S. 5:66, 68 Arberry
Yet these assertions introduce several problems. First, there are many issues in which the Quran does not confirm the previous revelation, but expressly contradicts it (e.g, contradicting the laws in the Torah, the core of the message of the Gospel, in addition to many smaller contradictions between Quran and Bible). Second, how can Muhammad judge Jews and Christians by the Quran and then expect these groups to judge by their own Scriptures, not by the Quran? How can the Jews and Christians perform what is commanded in the Torah, the Injil and all the rest of Gods revelations when the Quran contradicts many of the teachings and injunctions found in the Holy Bible? After all, what happens when a command of the Quran contradicts an express directive of the Torah and the Injil? What are the Jews and Christians supposed to do when this occurs? Are they to follow what the Quran says, thereby contradicting the Qurans own command to follow the directives of their own religious Scriptures? Or should they follow the guidance and light of their own revelation and simply ignore the Quran? Third, Muhammad himself is commanded to consult the previous Scripture if he has any doubt regarding the Qurans veracity:
If thou wert in doubt as to what We have revealed unto thee, then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt. S. 10:94 Y. Ali
This presupposes that the Bible is the criterion over the Quran, and therefore implies that the Bible must determine whether the Quran is true or not, what parts can be believed and which should not be accepted etc. This therefore means that every time that the Quran contradicts the instructions and prohibitions of the Holy Bible then the Quran must be rejected.
Thus, if Muhammad had passed judgment that contradicted the Torah and Injil then the Jews and Christians would be forced to reject his decision since this is what the Quran says they must do!
Here is a summary of all the problems that arise from what the Quran teaches so as to help the readers see more clearly the major dilemma caused by the Qurans conflicting instructions:
Contradictions in the Qur'an
Answering Islam Home Page