A primary distinction between the Islamic and Christian anthropologies is the way they interpret the frailty of human nature and likewise what is to be done about it.
In the Koran, we do not find a ‘fallen’ man, powerless against ‘sin’ and in need of redemption before he can gain access to salvation. Here “[God] created man in the best of stature.” Man has a nature that is what it is and has not been permanently damaged or altered, although it is susceptible to forgetfulness about its origins and therefore about God.
Man possesses knowledge of the Divine already, he need only remember it and cultivate the truth in order to be saved. Soteriology for Islam revolves not around the sacrifice of a lamb but around knowledge of the truth.
One way of describing this difference of point of view is to say that Islam envisions man as intelligence rather than will, while Christianity tends to emphasize man as will. The resulting views of human frailty correspond to this emphasis: in Christianity the will is weak and so many falls into sin and cannot ‘do what is right’; in Islam far less emphasis is placed on action and frailty of will, and it is the intelligence that has been weakened through ages of neglect, and so man does not ‘know what is right’.
It is evident that lack of knowledge can be rectified by a Messenger, hence Muhammad; but a broken will is hopeless without some kind of supernatural intervention, hence Christ the Redeemer.
Since man, in Christianity, is will, then to disobey is the great sin, and this makes Christianity moralist from the start; since man, in Islam, is intelligence, the great sin of Islam is the forgetting of God.
The ultimate and unforgiveable sin of Islam is ‘association’ or the making of additional gods or the setting up beside God equals, the confusion of the Absolute with the relative. This intermingling is shirk, the ultimate vice, because it is the direct denial of tawhid, the Oneness of God, which his the essential message of Islam.
 Koran, 95:4.