Problems with Christianity
Islam's view of Christianity is that it started off as a religion based in Jewish tradition but accepting Jesus as a prophet and teacher. In time these teachings got replaced by corrupted teachings that Islam rejects: Jesus was not God incarnate and he was not God's begotten son. Islam's view of other religions generally is based on them having received, at some point, prophets which taught the pure monotheism of Islam along with fundamental concepts of the religion such as the nature of sin and forgiveness. These religious teachings have become forgotten and corrupted over time and so God renews His revelation again and again until in the final revelation the message is preserved intact. This is taken to be the revelation given to Muhammad primarily in the form of the Qur'an.
There is much more to Islam's view of Christianity than I shall detail here, however my concern is only to identify the sin of disbelief in Christianity and to compare and contrast this with Islam.
It is quite hard to speak of Christian beliefs without being inaccurate because there is a large variety of sects. What I am referring to here is the form(s) of Christianity that I am most familiar with: namely mainstream Catholic and Protestant Christianity.
In Christianity the disbelief in God and disbelief in revelation are also key sins. They are however somewhat secondary to disbelief in the resurrection of Christ. As a Muslim, I believe Jesus to be a prophet and a great teacher who brought great evidence in the form of miracles. I try to follow what he taught in so far as I trust the sources through which I find out what he taught. This, however, does not make me a Christian. To be a Christian I have to believe that Jesus died on the cross to save Mankind from their sins; that Jesus was God incarnated as a man and that God is Trinity rather than Unity.
It is the acceptance of these doctrinal points that makes one a Christian. If you don't accept these you are not a Christian. (This at least is the definition of Christian that I shall be using.)
Historically many people called themselves Christians who did not accept these doctrinal points and today many people consider themselves Christians but have never really thought about these points. How these doctrinal points came to be part of mainstream Christianity is not my concern here. What matters is that they are incompatible with the sin of disbelief as it has been set out in the previous pages.
The aspect of the sin of disbelief in God connected to the purposiveness of existence (i.e. if existence is purposeless then everything you do is futile and worthless) is a line of argument that still applies with Christianity because in broad terms Christians consider themselves monotheists and believe that there is only one judge. Some Christians, however, may think of God as essentially two judges or maybe three with Jesus playing the role of lawyer who needs to be persuaded of your case before he appeals to God on your behalf. In Catholicism the graves and images of Saints were and are worshipped and asked for favours etc., These things pollute otherwise pure intentions by appealing to different judges, who in principle may judge by different criteria. This damages or destroys the idea of universal morality and absolute rights and wrongs. If there are many criteria there are many purposes of the universe and you choose which purpose to work towards. No deed of someone who believes in many judges can be said to be good or bad in absolute terms.
This is strongly connected to the concept of salvation through Jesus dying on the cross. By this act all the sins of Christians are supposed to be forgiven. This great act must have changed something about the way to salvation, i.e. that before the act people had a certain route to salvation and that after the act the route to salvation is profoundly different. However, if God fundamentally changes the way he judges people in different times from being harsh to being easier, then this can hardly be justice! Is this a change in God's justice or is it rather two judges: God the father and God the Son. On the other hand, if there is no fundamental change in the route to salvation, then why all the fuss? -it doesn't really matter whether Jesus died on the cross or not; there has always been one justice and one judge.
The aspect of the sin of disbelief in God being the ultimate explanation of existence is a line of argument that might apply to Christians but generally doesn't. The problems again lie in the paradoxes of the Trinity.
Essentially both religions assert that they believe in a God whose nature is beyond the human mind to comprehend fully. There is however a significant difference in the perspectives of why God cannot be fully comprehended by man. In the Islamic perspective the metaphysical existence believed in which includes as its core God is called 'al-ghaib' which basically means 'the unseen'. This specifically refers to observation rather than the sense of "I don't see that" meaning, "I don’t understand that". For example it could be said that the sun, when it is not visible to us at night, is part of 'al-ghaib'. Metaphysics in Islam is unknown essentially because of the limitations in our sense perception. In contrast to this Metaphysics in Christianity is unknown largely because our natural reason cannot understand it. God is 'above' logic.
At the heart of Christian doctrine is a profound mystery of paradoxes (e.g. everything belongs to God, He has complete power over everything. So in what sense can God sacrifice something? What does He give up? How can God be all knowing and at the same time not know what is going to happen? How can God become a man? See also "God made flesh?")
The effect of this is that the sin of disbelief in Christianity can't use a deliberate breakage of basic logical reasoning as a foundational element in the sin of disbelief. Christians don't merely 'not always expect better explanations' to their questions about reality, they also have no problems with explanations that are logically self -contradictory or paradoxes.
In conclusion the sin of disbelief in Christianity, because of its doctrinal beliefs, is far from the description set out above. Believing Christians can be very bad thinkers and people well informed about Christianity and extremely good thinkers may never become Christians. To illustrate the point I'll relate an incident that I heard about recently:
A woman brought up as a Christian accepted Islam and after some time decided to tell her mother of her decision. Her Christian mother asked why she had become a Muslim and she replied "Islam makes more sense" to which her astonished mother replied:
"But religion is not supposed to make sense!"