Judaism, Christianity and Islam are referred to as the three Abrahamic faiths because the three draw roots to the same patriarch, Abraham. Because of this lineage link, many also draw the conclusion that believers in each of the three are believing in and worshiping the same God. This is a false assumption. To understand why this is false, we must understand who each of these three religions worship.
In orthodox Christianity, we understand God to be the eternally-present, uncreated creator of all existence, of singular substance but represented in the persons of God the Father, God the Son and God the Holy Spirit. These three persons of God while having separate roles and relationships to one another are co-equal, co-eternal, co-existent. These are not three Gods, but one God existent in three persons. The triune nature of God we may never fully understand until we are at last with Him in the kingdom.
Jews understand God to be who He portrays Himself to be in the most simple of understandings of the Old Testament, which is as a singularity in person-hood. That is to say that Jews only worship God the Father, with disregard for both God the Son, who is foretold by Old Testament prophets, and God the Holy Spirit, who is present throughout much of the Old Testament. In denying the deity of both Jesus and the Holy Spirit, the Jews deny critical portions of who God is. So while they worship a concept of God, they do not worship Him fully, which makes that which they worship an idol of the true God.
Similarly, Muslims, who also deny the deity of Jesus, deny God His fully glory. But more basically, Muslims do not even worship the same concept of God, as Jews and Christians share. Allah, the name of the Islamic God, started his career as a minor moon god, one of many worshiped by the Bedouin people of the Southern Arabian peninsula. A full study of Islam reveals to the undertaker that it is a man-made faith concocted by a man in the late 600s AD in response to both the vacuum of power left by the collapsing Roman empire and the growth in pre-Arabian nationalism. The inventor of Islam attempted to graft the new religion into the Jewish and Christian traditions by usurping many of the people, places and events recorded in the Old Testament, but then perverts the Christian faith by denouncing specific tenets central to orthodox Christian doctrine. We also see this tactic used by the inventor of the Mormon religion.
A great tragedy is that Jews and Muslims are earnest in their beliefs, having been fully deceived by the lies of the adversary. The greatest tragedy, though, is that universalism has crept into the Christian church. More and more Christians are under the delusion that Jesus is not the only way to salvation. So, there is a growing propensity for some Christians to embrace this false brotherhood of the three faiths. But to accept that we serve the same God is to reject the deity of Christ. If we don’t stand for Biblical truth, then there will be no church to profess the truth to the blind and deceived. If we don’t stand on orthodoxy, then we pervert the Word. If we don’t live by the Word, we perish eternally.
We Do Not Serve the Same God