Why Are There Differences in Beliefs

There are countless numbers of 'religious' denominations. If many of them are reading from the same book, why are there so many variations of 'the truth'. Many groups teach that if you don't adhere to their teachings, then you have been led astray, perhaps by a demonic influence. If we are all created by the same being, why are there so many differences in beliefs in how we are to serve him, and why is not clearer. Many people believe with all their heart, soul, and might they are living as the spirit leads them, so who are we to judge what spirit they are being led by.