But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed. Galatians 1:8-9
There is no doctrine of original, Adamic or inherited sin in Islam. Without this biblical hamartiology (doctrine of sin), there is no biblical soteriology (doctrine of salvation), Christology or theology proper. A deficient doctrine of sin always results in a deformed doctrine of salvation. Without an actual Fall, salvation is reformulated as mere moral effort and a substitutionary Savior is seen as superfluous.
Galatians is an excellent place to start in analyzing the claims of Islam. Not only does it open with an important evaluation of angelic visions such as Muhammad received (Galatians 1:8-9), but it goes on to explicitly reject the Islamic exaltation of Ishmael over Isaac as the promised son (Galatians 4:21-31).
Additionally, it gives explicit testimony to the Fatherhood of God (Galatians 1:1, 16; 4:6), the death and resurrection of Jesus Christ (Galatians 1:1, 4; 3:1), the covenant keeping nature of God (Galatians 3:15-29), and an enslaved flesh in fundamental rebellion against God (Galatians 5:16-24) — all of which are clearly rejected in Islam Even more helpful is the epistle's emphasis on justification by faith apart from works.
The same book, which opens with a curse upon any who preach a false gospel, vividly describes the essence of all false gospels. The bulk of the epistle deals with the rejection of the gospel of grace and its relationship to faith. Paul relates a rejection of justification by faith to an anti-gospel. Where salvation by grace through faith in the person and work of Christ is opposed, works-based righteousness always springs forth. In effect, all other religions reveal this same tendency.
Whether the disciplined life of a Buddhist, the baptism and good works of a Mormon, or the prayers, professions and pilgrimages of a Muslim, all non-gospel religions are founded upon man's attempt to appease God through works. Islam, by its foundational rejection of salvation by grace alone through faith alone, reveals itself as a type of anti-gospel.
A Gospel of Faith or Works?
Because man is inherently unable to perform all the law requires (through original sin), he abides under a curse. Only a gospel which exalts one who is cursed in our place can save us from this curse (Galatians 1:10-14). All who teach a gospel contrary to the life, death, resurrection and future return of Jesus Christ are accursed because only in Him are we redeemed, reconciled and released from the curse Christ either bears the curse for you, or you abide under the curse yourself. There is no other option.
Justification by faith is essential to the gospel of Jesus Christ "If justification were through the law, Christ died for no purpose" (Galatians 2:21). "You are severed from Christ, you who would be justified by the law" (Galatians 5:4). Islamic theology holds no place for the biblical gospel of salvation by grace through faith. Islamic justification is simply an arbitrary work of a capricious God taking into account a person's meritorious discipline and duty. This is not the gospel of the Bible. This is not the true gospel.
Muhammad received an angelic vision in a cave in Mecca, but angelic visions are not inherently authoritative for "even Satan disguises himself as an angel of light" (2 Corinthians 11:14). All visions, prophecies and revelations must be assessed by the canon of Scripture, for all which oppose the biblical gospel are false and accursed (Galatians 1:8-9). In light of this standard, Islam is weighed and found wanting.