Last night, there got to be several interesting discussions going on on the comments here. One very interesting question was asked, which I'm going to have a go at beginning to answer. I've edited the question heavily, and the setter can feel free to have a go at me about that.
With all the different mutually contradictory faiths in the world, why should one trust Jesus (for example) rather than anyone else? Isn't it simpler simply to say that none of these extraordinary claims are true?
Why does God/Jesus not just speak to each of us individually at a certain point in our lives, or maybe at a number of points, and explain the 'rules of the game'? God doesn't seem to do this, so surely it's not fair to expect people to follow what the Bible says if they don't know it's the truth.
I think it's worth being clear, first up, that the key issue here is trust, and how we decide who we trust. Christianity isn't about doing the right thing and helping old ladies across the street and whatever. It's about trusting Jesus. And yes, trusting Jesus should then make a difference in the way that we live, but it's the trust that makes the difference between someone who is a Christian and someone who isn't.
I think there are a couple of things to look for when deciding who to trust. Sorry folks if this sounds obvious at times...
1. Are They Trustworthy? - Motivation
How do we assess whether people are trustworthy? One important way is to look at what their possible motivations were for saying what they said and doing what they did. If there's a good chance they were motivated by something other than truth, then it makes it less likely they were actually telling the truth.
For example, L. Ron Hubbard, the founder of Scientology, is quoted as saying (many times, variations, etc) "The best way to get rich is to start a religion." He started a religion; he got rich. Is it a trustworthy religion? Probably not.
Mohammed was a great military leader. Islam served very well to unite his followers, give them something to fight for and a passion to fight for it. Mohammed did very well out of it and conquered a large chunk of the Middle East. Is what he said to unite the people trustworthy? Probably not.
David Berg founded a (pseudo-)Christian sect called "The Family" aka "The Children of God". One of their core doctrines was that since everyone was united with everyone else, any two members could have sex (with permission of the leaders) and the leaders could have sex with anyone they wanted to, with or without permission. After he died, there were a huge number of accusations of sexual molestation against him. Was his teaching trustworthy? Probably not.
Jesus Christ's teaching centred on the importance of his death. During his ministry, he lived as a homeless guy (though sometimes staying with friends and followers). Then he was betrayed, beaten and horribly executed. On a human level, what did he have to gain from it, unless he believed that what he was saying was true? His key followers, likewise, gave up their (in some cases very profitable) livelihoods, travelled around the world telling people, and in most cases were also horribly executed. What motivation could they have had, unless what they were saying was the truth?
I'd strongly strongly recommend that anyone looking at whether Jesus can be trusted should read one of the contemporary accounts of his life in a modern translation. Here's an online one.
2. Are They Trustworthy? - Consistency
If there is a "true religion", then we'd expect the beliefs to be both internally consistent and consistent with other truths about the world. So if a religion taught that the earth was formed from the head of a dead giant (Roman paganism, IIRC), then it's probably not true. If in the past the scriptures said that the Holy Land was Tennessee, and then when it became illegal there, they were changed to say Utah (Mormonism, IIRC), it's probably not true.
Note that this doesn't affect claims to miraculous events where gods are involved. After all, if a being exists who is so powerful that science just describes the normal way he runs the universe (which is the proper Christian belief), then they can do things which science would normally forbid. It does, however, affect miracle claim where there is no mechanism proposed, and no gods involved to do the miracles (as in Buddhism, for example).
3. Are They Trustworthy? - testable predictions / claims
Some religions make testable predictions or claims. For example, the offical body of Jehovah's Witnesses, which they believe always to be correct, has often predicted the end of the world in specific years or on specific dates (details here). They've always been wrong. Are they trustworthy?
Of course, if a religion predicts or claims obvious or easy things, or things that are so vague they could mean anything, then that doesn't help with testability.
Christianity does make a few about the future. It could certainly be well argued that Jesus (died c. AD33) predicted the destruction of the Temple in AD70. It also makes the remarkable claim that Jesus rose bodily from the dead on the Sunday after he was killed. There's plenty of evidence for it, and I've never seen an even vaguely plausible argument against it. There's a useful quick summary of the arguments for it here - it's something you can look into.
4. Are They Trustworthy? - worth trusting?
Another question is of course whether it's worth trusting them at all. What do you stand to gain or lose?
For example, I have trouble understanding Hinduism and Buddhism. Part of the problem is that they're never actually explained in English - it's always things with stupidly long non-English words in. Even when Wycliffe was translating the Bible into English and the right word didn't exist, at least he made up words that made sense from the root langauges. But one thing I do understand about Hinduism and Buddhism is that there seems to be very little to gain if they are true. I enjoy life. By God's grace, I rejoice even when stuff hurts and people die, even when there's pain, I enjoy knowing who God is and I'd rather be alive than not. So a religion than aspires to nothingness, or that says you get an unlimited number of other goes anyway somehow just doesn't seem worth following.
I think that's more than enough writing from me for now, and I still haven't answered about whether it's fair. But I hope I've had a go at the first few questions.