Recognizing the inherent limitations of speaking for another faith tradition, we can say that Judaism and Islam differ primarily from Christianity in an understanding of the person of Jesus. For Christians, Jesus is the fulfillment of the Old Testament promise. Judaism and Islam simply do not recognize the divinity of Jesus. For this reason, the monotheism practiced by Judaism and Islam is different from the monotheism of Christianity. Christianity has a Trinitarian understanding of the one true God because of Jesus’ divinity that is not shared by Judaism and Islam. In fact, Islam would describe Christianity as a tri-theist religion, rather than a monotheistic religion.
When it comes to reading the Old Testament, a Christian reading necessarily requires the lens of Christology, whether implicitly or explicitly. And yet, the Old Testament must be viewed in continuity with the New Testament, since both are of the Word.
Judaism, on the other hand, does not share the belief in the Incarnate Word. The Old Testament is read not with Christ in mind, but as the earliest inspired documents of their religious community through which they can see and better understand their present existence as a people.
Finally, an Islamic reading of the Old Testament would be very different from both a Christian and Jewish reading. Islam thought of the Old Testament as becoming somehow defective. Therefore, any reading of the Old Testament would be a Koranic reading since Muslims believe the Koran is the true word of God that actually supplants the Old Testament. Put another way, if a contradiction existed between the Old Testament and the Koran, the teaching of the Koran would be adhered to while that of Old Testament would be rejected.
The person of Jesus Christ and the meaning of his passion, death and resurrection are viewed differently by all three religions. For the Jewish people, Jesus cannot be divine because there is only one God, and for them his divinity presupposes the existence of two. Their Shema commands: “Hear O Israel, the Lord is our God, the Lord is one” Deut 6:4. For Islam, too, the concept of a Trinitarian God implies the existence of more than one God. Both Judaism and Islam acknowledge that Jesus was a great prophet, but deny his divinity. Consequently, one basic way that a Christian reading of the Old Testament differs from that of Judaism and Islam in that Christians see another level of meaning in it, namely a Christological one which interprets texts in the light of the paschal mystery.