It has often been said that all religions teach basically the same thing, or are just different paths to the same God. This idea of religious pluralism comes both from the mouths of those who have made a career of “religious studies” and from those who simply mean to brush aside the whole question of truth in religion. The claim itself can be answered in several ways, but I think that behind this idea stands a common presumption about the nature of God and what He expects from us.
For most, this claim is simply a matter of ignorance about what the various religions actually teach, or consider to be essential truths, but when the most foundational doctrines are taken into account irreconcilable differences immediately surface. For example, Islam says that Muhammad was the final and greatest prophet of God and that Jesus was a mere human prophet, while Christianity says that Muhammad was not speaking for God and that Jesus was actually God incarnate. Islam claims that the Christian’s divine view of Jesus is a mortal sin (the sin of “shirk”), while Christians say that you must accept His deity for salvation. There is no harmonizing these views. Other examples that could be cited would be the Buddhist idea of a non-personal God (in fact, Theravada Buddhism is essentially atheistic) vs. the eminently personal God of Christianity; or the Hindu/New-Age idea of reincarnation vs. the one-life model of Christianity.