In Sura 4 of the Qur’an, verses 47-48, we read: “O ye People of the Book! Believe in what We have (now) revealed, confirming what was (already) with you, before We change the face and fame of some (of you) beyond all recognition, and turn them hind wards, or curse them as We cursed the Sabbath-breakers, for the decision of Allah Must be carried out. Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed.” [The Kindle Qur'an (Definitive English Edition), Islamic Studies Press.]
According to the above Qur’an passage calling Christians (“People of the Book”) to believe the Qur’an, warning us of certain consequences if we do not, to “set up partners” with God is the unforgivable sin. Muslims believe Christians have “set up partners” with God—that we are actually not monotheists. They believe we blaspheme God by worshipping Jesus, and the Holy Spirit. The Qur’an says Christians are lost in sin due to believing in the deity of Christ. Needless to say, Christianity and Islam aren’t compatible. They’re mutually exclusive religions. If one is right, the other is wrong.
Do we who worship Jesus “set up partners with God”? Are we monotheists or not? I believe the above Qur’an passage is from a misunderstanding of the doctrine of the Godhead (or Godhood). We don’t believe in three gods! We believe in one God who exists in three persons—Father, Son, and Holy Spirit (Matthew 28:19). Jesus said, “I and the Father are one” (John 10:30). The Father, Son, and Spirit are one God! Muslims will sometimes argue, “No man can become God.” They think we’re saying Jesus, born a mere man, somehow ascended to godhood. The truth is, God the Son became a man. Can an omnipotent God not take on flesh if he so chooses? There’s only one God, and he has no partners in Godhood, but the nature of Godhood is more complex than human nature. No one exalted anyone into Godhood, but rather, one person of the godhood “emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. Therefore God has highly exalted him and bestowed on him the name that is above every name, so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father” (Philippians 2:7-11 ESV).