The Qur'an argues in various passages that it is from God BECAUSE it is in harmony with the earlier scriptures. It calls the Jews and Christians to b elieve in the Qur'an BECAUSE it says the same thing as the earlier prophets. This is a proper scriptural principle. It is the same as in the Bible, later prophets are to be judged on the basis of the already established revelation. You cannot judge the former by the latter.

Muslim response: This is silly. So as a Christian, you would never think of using the Bible to judge the truth of the Vedas?

This comparison has nothing to do with the issue. The Bible at no point claims harmony with the Vedas. It does not say it is a successor of the Vedas or that it comes from the same source as the Vedas. The Qur'an on the other hand says that it comes from the same source as the revelations to Moses, David, Jesus etc. i.e. the prophets sent to the children of Israel. The Qur'an appeals to them to believe its message *because* it is in the same tradition and in harmony in its message. That is the reason Muhammad expects to be believed. Therefore there is all reason to measure the Qur'an by its own claim.

No claim connects the Vedas with the Bible.

But this is very much a claim of the Qur'an and based on this claim and the fact of irreconsilable contradictions between the Qur'an and the Judaeo-Christian scriptures, the Qur'an fails its own test for divine origin.

Surat-ul Maida (5):49-50 exhorts the Christians:

And in their footsteps We sent Jesus the son of Mary, confirming the Law that had come before him: We sent him the Gospel: therein is guidance and light, and confirmation of the Law that had come before him: a guidance and an admonition to those who fear God.

Let the People of the Gospel judge by what God has revealed therein.

And we do exactly that, and this is the reason we have to reject the Qur'an which contradicts the Gospel.