There are long books answering questions like these, and but this short quote from Vern Poythress gets to the heart of the matter:
Are All Religions True?
Let us consider one common difficulty that modern people have with the Bible: how can there be only one true religion?
People ask this question partly because they are aware of multiple religions—Christianity, Islam, Judaism, and Hinduism, to name a few. How do we respond to this multiplicity? One person, whom we may call Sue, concludes that all religions are equally right. She says that they all have a common core having to do with a loving God and being kind to your neighbor. But in selecting a common core Sue shows her own personal religious preferences. Sue speaks of a loving God. But Buddhism does not believe in a personal God. So Sue has excluded Buddhism rather than being all-inclusive. She has also excluded polytheism, which believes in many gods rather than one. Sue speaks of being kind to your neighbor. But some religions have practiced child sacrifice (Deut. 18:9–10).
When Sue talks about a common core, she has also put into the background the irreconcilable differences between major religions. The Bible teaches that Jesus is the Son of God. The Qur’an says that he is not the son of God, but only a prophet. The New Testament part of the Bible teaches that Jesus is the Messiah promised in the Old Testament. Modern Judaism denies that he is. Sue implicitly disagrees with all of these convictions when she implies that they really do not matter. Christianity, Islam, and Orthodox Judaism all exclude one another by having beliefs that are denied by the other two. Sue in practice excludes all three by saying that the exclusive beliefs are not the “core.” Tim Keller observes, “We are all exclusive in our beliefs about religion, but in different ways.”
Are All Religions False?
Let us consider another example. Donald looks over the field of religions and concludes that they are all wrong. He thinks that they all make arrogant, overreaching claims to know the truth. The differences between the claims show that no one really knows. Donald’s position is just as exclusive as Sue’s, and just as exclusive as the claims of any one traditional religion. How so? He claims to know better than any religious practitioner the true status of religious claims. But you have to know a lot about God—whether he exists, whether he reveals himself, what kind of God he is—to make a claim that excludes all religions before seriously investigating any of them in detail. Donald thinks that religious claims are arrogant. The irony is that he is acting arrogantly in claiming to be superior to all religions.
[This book just came out. Vern Poythress, Inerrancy and Worldview, Crossway, 2012 (p. 19-20)]