Short Answer: Because God commanded us to respect them. They have deviated from true Monotheism, but we still have much in common, including our love for the same prophets.
Asalamu Alaikum Abdul Rahman. Thank you very much for your question.
Both Groups Deviated From The Truth
It is true that all the prophets of Allah preached the same religion and all their followers were originally Muslims (those who submit their will to God’s will).
And it is also true that after the times of the prophets of the past, most of their followers deviated from the true religion taught by their prophets.
This is obvious in the case of contemporary Judaism and Christianity.
Some Jews ignore the fact that the oneness of God implies the oneness of humankind; they distort the idea of the chosen status of the Children of Israel to misappropriate an exclusiveness for divine grace and to claim racial superiority.
The followers of Jesus split up into two sects immediately after the disappearance of Jesus, and one of them eventually eliminated the other.
The sect that prevailed followed St. Paul, who over-ruled many of the fundamental teachings of Jesus.
As a result, we have Christianity preaching trinity, original sin, and atonement through the suffering and crucifixion of God’s “only Begotten son”.
But, We Still Have Much in Common
We can see that both Judaism and Christianity are not now the original religion taught by their prophets.
But at the same time, there are many beliefs and traits of the original religion of God still existing in those two religions.
It is a fact that these two religions are closer to Islam than any other religions.
So, there is a strong case for our appealing to the followers of these two religions, considering the common factors they share with Islam.
For instance, they believe in all the prophets of God whose stories are narrated in the Old Testament.
The Torah of the Jews contains several verses originally revealed to Prophet Moses (Musa—peace be on him), and the Gospels of Christians contain several verses revealed to Jesus (peace be on him).
Similarly, the stories of Prophets Moses and Jesus (peace be on them), as given in their respective books, also carry a lot of truth.
We know this from our own Noble Quran, which is the Criterion (Furqan) by which we judge what is true or not.
Allah Commands us To Respect Them
It is most especially noteworthy that in spite of all the deviations of the Jews and the Christians, Allah Almighty calls them “The People of the Book” and commands us Muslims to address them as such.
So, we must give the Jews and the Christians special respect and consideration, despite the differences.
The Quran says:
Say: “O People of the Book! come to common terms as between us and you: That we worship none but Allah; that we associate no partners with him; that we erect not, from among ourselves, Lords and patrons other than Allah.” If then they turn back, say ye: “Bear witness that we (at least) are Muslims (bowing to Allah’s Will). (Quran 3:64)
And Allah knows best. I hope this helps answer your question. Please keep in touch.
(From AboutIslam’s archives)