(Edited and expanded on September 7, 2013)
As my immediate family and closest friends already know, I decided last year to reject the Christian world view. My reason is simple: not enough evidence. The Bible is not consistent with itself, let alone with what we can observe of reality, including history, science, and the reality of suffering all over the world. There is no strong evidence that Jesus is who the Bible claims he is, or that he rose from the dead. Some Christians point to a personal experience of positive change in their lives as evidence. But that doesn't lend any weight to Christianity, because people of all religions claim to have experiences which they interpret as validation of their religion. And faith, which is belief without sufficient evidence, is a cop-out.
Of course, these are just assertions, and my rejection of Christianity would not be rational if I didn't back those assertions up. The nice thing about the Web is that I can simply link to some more detailed treatments of these subjects. So here are a few links to get you started. I may not agree with all of these authors on every single point, but I'm in substantial agreement with them.
Yes, I'm unabashedly linking to the writings of vocal atheists, and I've read many more such writings than I've linked to here (along with Christian apologetics). My Christian parents and other family members have expressed their concern that I've opened my mind to the influence of the devil, and have asked that I refrain from doing so. I agreed to refrain for a time last year, but not anymore. I believe that the very idea of Satanic influence is simply a way that Christians silence skepticism, so that they can feel all right about not questioning their own beliefs, and so that the Christians who claim a position of leadership (priests, ministers, pastors, etc.) can retain their power to tell the rest of us how to live our lives. If Christianity were true, then my belief should have withstood my study of arguments from both sides.
Some Christians may claim that I am rejecting Christianity because I want to live a selfish, immoral life, or because I don't want to be accountable to my creator. That is emphatically not the case. I'm not perfect, but I want to live an upright, moral life. In fact, if you ever notice that I'm using my new beliefs to justify an act that is clearly immoral or overly selfish, then please call me out on it. Furthermore, if there is evidence that we humans were created by some intelligent being, that this God is still alive, and that this God cares how we live our lives, then I'd love to receive guidance from this God. But I don't know of any compelling evidence for such a God. Instead, we have many contradictory claims that people have made about God; not only do we have multiple religions, but Christianity itself is divided on numerous points, and the canonical scriptures of Christianity are contradictory. So it seems foolish to organize my life around the belief that the Bible is true, whatever that means.
So what do I believe instead?
At the most basic level, I believe that beliefs should be backed by logic and evidence. In other words, beliefs should be internally coherent and should be consistent with observable reality. What are my logic and evidence for this belief? Science, which is based on logic and evidence, has improved our lives dramatically over the past several centuries; we know that it works. And we use logic and evidence to determine the truth or falsehood of ordinary factual claims. So why should we grant privileged status to a set of writings that are supposedly the word of God, as Christians say we should? Why should we automatically accept these writings as truth? Why not discover whether these writings are true, based on logic and evidence, as we would for anything else? To do otherwise is inconsistent. The way I see it, the Christian exhortation to accept the Bible as truth, overriding reason, science, and everything else, is just another way of silencing skepticism.
I believe that morality is all about maximizing overall well-being for all conscious beings, most notably humans but also including many animals. No human is smart enough to really pull this off, so for the most part, we have to rely on rules to approximate this goal. On a more personal level, I believe it's fine to pursue one's personal goals, but only to a point; we need to be considerate of others. In particular, we need to avoid doing to others what we would not want done to ourselves. Many moral guidelines can be derived from these principles. Some moral choices are pretty clear-cut; some are vexing. And it seems to me that the Bible is of little value as a guide to morality. Of course, I'm not setting myself up as some kind of authority on morality; this is just my current understanding. And I'm still working on practicing these principles in my own life.
The topic of origins has been notoriously contentious, but I don't believe that it's as crucial as many make it out to be. I'll grant that there might have been a creator, who set up the universe, tuned it such that life could develop in some parts of it, and got the process of evolution started on Earth. Maybe the creator even intervened in the case of humans, to give us consciousness and intelligence, while not caring that our bodies are suboptimal in many ways because of evolution. But it doesn't automatically follow that the creator is the Christian God, and that I should therefore go running back to my old religion, automatically accepting everything the Bible says is true. If the Christian God is real, I need reasons to believe that, not just reasons to believe that there must have been a creator.
We can't know for sure what we will experience, if anything, after we die. However, there is overwhelming evidence that intelligence, memory, and personality are all dependent on the brain. All of these things can be seriously damaged when a brain is injured. Therefore, it's reasonable to believe that when a person's brain stops functioning altogether at death, that person ceases to exist. So heaven is probably just man-made wishful thinking. And hell, a place that some people I love are now worried that I will eventually go, is just something that someone thought up to scare people into submission. That strategy has proven to be an effective way of spreading more than one religion, but it's irrational to decide what to believe on the basis of fear. For starters, I don't think it's possible to "decide to believe" something unless one is actually convinced that it's true; at best, I could pretend to believe, and a God who knows my heart and decides my eternal destiny based on what I believe would see right through all pretense. Besides, more than one religion claims eternal damnation in hell for those who don't believe, so which one am I supposed to believe? The one I was raised in? No; I insist that logic and evidence are the only reasonable foundation for our beliefs. So both the promise of an eternal reward and the threat of eternal punishment are irrelevant to me. Though I can't be certain, I think it's most likely that when I die, I will cease to exist. What I believe in the meantime will be based on logic and evidence, as I understand them, not on hope or fear of an afterlife.
I believe that, in the absence of evidence for a god or an afterlife, we should live as though we're on our own and this life is all there is. This, I think, is where my beliefs collide most directly with Christianity. Christians believe that this world is ultimately doomed, and that it's most important to prepare for eternity, meaning the world and the life after this one. Frankly, I'm afraid that if the first part of that belief is taken to heart, it may become a self-fulfilling prophecy. A belief that God will eventually set things right also seems to encourage apathy about the state of this world; at least it did that in me for a time. I now believe that this world and this life are all we can be certain that we have, and when we see something wrong with this world, we should consider what we can do, if anything, to make it better. In particular, in a country like the US that has at least a semblance of democratic government, it really matters whom we vote for and what causes we support. We should all try to make this world a better one for all of us. It sounds trite, but I truly believe it's the best goal to which we can aspire.