All people, regardless of religious label, are guilty of holding the idea of exclusivity when it comes to their faith. Even if their practice is nonexistent, they would feel that their religious label is above those of other people, and become greatly disturbed when a family member leaves the faith and enters another one. One rather bizarre example of this can be found in an example related to me by a Muslim sister. Upon her acceptance of Islam, the sister's aunt became upset, and even struck her in the head with a Bible. Obviously, the Aunt had forgotten the various teachings in the book [aka the weapon] she yielded about forgiveness and mercy, this emotional reaction has little to do with religious sincerity, but rather family honor, embarrassment, and amazement that the Muslim sister had enough faith in God and the faith she has adopted to actually take steps to change her life from the prior manner in which she had been living.
This is not to say Muslims don't have these tendencies as well, indeed, these traits can be found among all people. The reason, in my view, is that we don't want to acknowledge that God's guidance is not restricted to "our own". The following Biblical verse is often cited to dissuade acceptance or even investigation of faiths other than the Christian sects.
Certainly this verse has been variously interpreted among Christians, but even accepting it at face-value, it does not negate God's message being continued and perfected by other teachers, Prophets and Messengers from Almighty God. If we accept one legitimate messenger of God, then we must accept them all. As Jesus was sent by God [that much can be accepted by Christians and Muslims alike], then the Biblical statement[if it's an authentic statement of Jesus] would still show the truth of our assertion above. Common sense should also come into play, if we believe in God, and accept him as the creator of the universe, of all peoples in our world, how can we say that God has favored one group of people over another, that he is un willing to send revelation, or communication, if you will, to figures other than those among the Jewish tribes in Palestine? That would indeed be senseless, especially in a world which, at that time, contained nations and peoples isolated from each other.
The Qur'an: the universal gospel
An honest reading of the Judeo-Christian texts and traditions show that its focus is mainly the Jewish people. The Old Testament, accepted by Jews and Christians as God-inspired, is mainly a collection of semi-historical narrative of varying accuracy. It does not seem to even allow the possibility of God giving spiritual guidance and assistance to those outside th Jewish people. While Monotheism [known as the 'Shema' to Jews and 'Tauheed' to Muslims] is certainly an important theme in the O.T., it still restricts God, whom they acknowledge as the creator of all nations, as a tribal or national deity!
The Qur'an, on the other hand, has from the earliest texts revealed acknowledged that God has sent guidance, in the form of messengers, Prophets and scriptures, to people everywhere. It has made acceptance of all messengers a fundamental principle, using very strong language regarding those whose acceptance is limited to one or two personalities.
Those who deny Allah and his messengers and [those who ] wish to separate Allah from his messengers, saying 'We believe in some but reject others', [those who] wish to take a course midway. Truthfully, they are Rejectors [of God] [Kaaferoon]....to those who believe in Allah and his messengers, and do not make any differences between anyone of the messengers, We [Allah] shall give their [due] rewards. Allah is forgiving, merciful [Q 4"150-152]
The above Quranic verse goes to the core of Kufr. That core is using religion as a national or cultural label, or viewing God as always being on their side. They don't want to be on God's side! This is not what God wants, and such behavior displays an actual rejection of God, their proclamation as believers is shown to be utterly hypocritical.
Muslim polemics often involve attempting to 'prove' that the Prophet Muhammad, upon whom be peace, is mentioned or predicted in the Bible. Numerous books and tracts have been produced expounding on this point. While, as a Muslim, I do believe much of these assertions to be accurate, it should not be necessary to prove it through these means. I hate to say it, but sometimes Christians have a point when they ask us the motive for our usage of Biblical texts while rejecting the bulk of it as inauthentic. Instead of focusing on the similarities between Moses and Muhammad, or explaining that the 'spirit of truth' [John 16:13] refers to him, we should convey the message itself. That message is that of a pure and uncorrupted monotheism, one that serves as a clarification and a correction of prior assertions as to the reality of God, his messengers, and the Divine message. For example, the Old Testament actually has a narration in which God wrestled with Jacob, in human form, and lost [Gen. 32:25]. We should be asking 'How can God loose? More importantly, does it make sense for the Creator of the entire universe to be engaged in a wrestling match, in human form, in the first place'?
The answers are all there, in the Qur'an! It asserts that God does not take human form, that God is a force that has neither beginning nor end, that God is dignified and Noble, and that God is the nourisher of all. [ft.1]
To have respect for God means to see his laws, his creation, in a much broader light. It means examining the message rather than scrutinizing the messenger! It means following what is right and true, regardless of the source. It means placing God as the uppermost priority in our lives, and by that we will receive blessings and support that is unimaginable.
 This issue has been dealt with in great detail in the Book The language of revelation. To obtain this volume, email firstname.lastname@example.org.