The question is asked. “Is the God of the Old and New Testaments the same as Allah of the Quran? The specific answer is NO! The foundational teachings of Islam are unequivocally contradictory to foundational Christianity!
Jesus spoke of Himself and God as follows: (Sonship) “The Father and I are One.”(John 10:30). “He who has seen Me has seen the Father.” (John14:9).”Why do you accuse me of blasphemy because I said ‘I am the Son of God?” (John 10:36). And: “I am the way, and the truth, and the life; no one comes to the Father, but through Me.” (Death and Resurrection) Jesus said: “… the son of man will be delivered to the chief priests, and scribes, and they will condemn Him to death, and deliver Him to the Gentiles to be mocked and scourged and crucified, and He will be raised on the third day.” (Matt: 20, 18.). (Forgiving Sin): Jesus said: “Her sins, which are many, are forgiven…” “Thy faith has saved thee.” (Luke 7: 47, 50.) Jesus said: “I said therefore unto you, that ye shall die in your sins; for if ye believe not that I am he, ye shall die in your sins.” (John 8:24.) (Eternal life in Jesus Christ) “And as Moses lifted up the Serpent in the wilderness, so must the son of man be lifted up, that whoever believes in Him may have eternal life.” (John 3:14) (Trinity) Jesus said: “All power in heaven and on earth has been given to Me. Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold I am with you always, until the end of the age.” (Matt: 28, 18-20,)
Mohammed disputes Jesus’ identity with God in the Quran: (Trinity): “Unbelievers are those that say: “God is one of three” There is but one God.” (5:73). “So speak of God and His Messengers, but do not speak of a ‘Trinity’ …God is only one God.” (4:171). “Those who say, ‘God is the Messiah, the son of Mary’, are defying the truth.” (5:17). “The Messiah, son of Mary, was no other than a messenger.” (5:75). “The Christians say that the Messiah is the son of Allah…may Allah destroy them.” (9:30). ). “Had God desired to take to Him a son, He would have chosen whatever He willed of that He has created. (5:75). (2) As to the death and resurrection of Jesus the Koran teaches: “And their saying (writer note: speaking of the Jews) : Surely we have killed the Messiah, Isa [Jesus] son of Marium, the apostle of Allah; and they did not kill him nor did they crucify him, but it appeared to them so (like Isa) and most surely those who differ therein are only in a doubt about it; they have no knowledge respecting it, but only follow a conjecture, and they killed him not for sure” (4:157) Nay! Allah took him up to Himself; and Allah is Mighty, Wise. (4:158) And there is not one of the followers of the Book but most certainly believes in this before his death, and on the day of resurrection he (Jesus) shall be a witness against them. (4:159.
Now: the physical death and resurrection of Jesus Christ for the sins of others is the foundation of Christianity-without physical death on the cross there is no Christianity or reconciliation with God-and it is denied, disputed and trivialized in Islam. (3) As to the forgiveness of sin: The Koran teaches no one can die for the sins of another: No human being, according to the Qur'an, is entitled to carry the sins of another before the judgement of Allah (Suras al-An'am 6:164; al-Isra' 17:15; al-Fatir 35:18; al-Zumar 39:3; al-Najm 53:38), for everybody has to ask for his own forgiveness from the Holy One. According to Islam, any substitutionary sacrifice is impossible. (Jesus teaches His physical death for sin was substitutionary and essential for believers.) But, in love, Jesus freely forgives and resurrects those that believe and trust in His provisions!
Neither is true God contradictory about our interactions with other humans. Jesus taught: ”…do to others as you would have them do to you. For this is the essence of the Law and the prophets.” (Matt 7:12) and “…love your enemies, and pray for those that persecute you.” (Matt: 5:44). Mohammed disputes Him and teaches: Arberry: "O believers, take not Jews and Christians as friends; they are friends of each other. Whoso of you makes them his friends is one of them. God guides not the people of the evildoers."