Introduction
There is some overlap between Christianity and Islam, however there are very key differences. The largest difference is our understanding of what God is and is like. Christianity claims God is merciful, but Islam does not. Logically, there is no reason why a Christian would consider the tenets of Islam more conformed to the truth the those of Christianity. That which we hold in common invalidates Islam because on account of the Lord, Lunatic, or Liar argument of C.S. Lewis, it makes no sense to claim Jesus was only a messiah and not the Son of God. There does not appear to be a great amount of study beyond this on these issues besides what you will find below. Nevertheless, is page is open to both people looking to find a case for Christianity as a Muslim, those looking to understand the Muslim faith as Christian, and perhaps those interested in talking faith with one group or the other.
Nature of the Problem
Filial vs. Servile Fear
Regardless of where you find yourself on this subject, it is important to note a key difference between Islam in comparison to any other belief system. Namely, the fear that Islam instills in those who believe it. This may seem like there is a negative connotation but be assured this is unintended. In Christianity and even in family life at large, there is a certain fear in the nature of love, which we shall call filial fear. Filial fear is the kind of fear that extends from the will to the good of the other and then from fearing to bring harm/offense to the beloved which motivates us to know and do the good which is abstractly and really desired by the lover. From the latter, there is another subset that motivates obedience to parents or leaders in that relationship, and this is not out of fear of punishment oneself but out of love and humility to serve especially those who go out of their way to love us. This is a Christian expression of a teleological aspect of love. In the Islamic understanding of God, some requirements are somewhat contrary to authentic human tendency such as the rule to never attempt to imagine Allah. This fear will hold the believer in a way that apparently convenient tenets of love which even the Christian faith holds let alone the less accurate Modern/Post-Christian culture holds. The problem here can be what can refute fear. Entertaining counterarguments is difficult unless they permit themselves to do so as a result of their own investigation since it is something of an all-or-nothing faith.
Our Major Figures
Christ had a gospel of love and chose against the destructive method of conquering, which his followers were expecting. Islam became popular likely in part by the expectation for an earthly savior. Muhamad, on the other hand, had a relatively militaristic mindset and this added even greater complexity because the non-believer is the enemy, and who listens to their enemy on their beliefs?
Moral Disparity
Due to the difference in our major authority figures, what is morally acceptable to Muslims has very different Muhammad-based morality. Muhammad was not a very moral person by Christian standards. The understanding of sin and the human condition is very different as well both by consequence and by the nature of the different narrative than Christians have.