A friend of mine recently asked me how I had come to change from being an atheist to a theist, a conversion he found unusual. This is a big subject but, I think, an interesting one, so I thought I would outline some of my reasons here. Please note that I am in no way saying that the points enumerated below constitute any sort of proof of God. I doubt there is any such proof, in the absolute sense, and even if there is, it would take far more than a few sentences to supply it.
Also, I might note that switching from atheism to theism is probably not that unusual at all. In fact, I would venture to say that many people go through a process of development something like this: As a child, the person believes unquestioningly in God because all the adults around him tell him that he should. Then as he becomes a teenager, he starts to doubt the adults and, in time, decides that God is just another scam like Santa Claus and the Tooth Fairy. Later, as an adult, he begins to consider the possibility that his childish conception of God was simply inadequate, and that a more mature conception may have merit. Eventually, for a variety of reasons, he finds himself a "believer" again, but not at all the kind of believer he was in his childhood.
I suspect that this course of development is very common, though I don't know of any studies that would prove it.
I also note that even a philosophically committed atheist can change his mind - as did Anthony Flew, the world's most famous atheistic philosopher, who not long ago announced that he had been persuaded to adopt theism, albeit reluctantly. Flew's reasons match some of my own, as you will see. Incidentally, I found it very courageous of Flew to make this announcement, when it would have been far easier for him to keep silent.
Okay, my reasons, in no particular order, and without much elaboration:
1. The anthropic principle and cosmic coincidences. It is now a commonplace of astrophyics and cosmology that our universe appears to be "fine-tuned" to be orderly and habitable. Either the universe is a product of design (the God hypothesis), or there are trillions upon trillions of universes (the "multiverse" hypothesis), and by chance one of them turned out just right. Either explanation requires a leap of faith, since neither God nor these parallel universes can be directly perceived. I go with God as the simpler and more elegant solution.
2. The origin of life. The old idea that the first living cell came together spontaneously by pure chance is no longer seriously argued, now that scanning electron microscopy has shown us the fantastic complexity of even the "simplest" cell. Some scientists speculate that a "self-organizing principle" was responsible, but no one has found evidence of such a principle, except in oversimplified computer models that bear little relationship to real chemical activity. Further, the origin of life involves the origin of information, since life is based on DNA, and DNA is essentially a means of encoding instructions on how to build and deploy proteins. Information is a qualitatively different thing from mere repetitive order (e.g., a DNA molecule is different in kind from a snowflake), and it is unclear how information could emerge spontaneously out of disorder or mere repetitive order. A Mind that supplied both the information and the structure necessary for the first living cell seems like a better bet than any purely naturalistic scenario.
(Points 1 and 2 are the ones that apparently persuaded Anthony Flew.)
3. All attempts to ground morality in naturalistic laws or brute physical facts have (in my opinion) failed, leaving us with two choices: either moral values are subjective and arbitrary, or they are objective but grounded in something outside nature. A world of purely arbitrary moral laws is one in which no one could condemn murder or the Holocaust as evil. I find such a condition intolerable, so I opt for ethical nonnaturalism (even though I concede that this position, too, is tricky to defend).
4. Materialism, the view that the physical world is all that exists and that mind is, at best, only an epiphenomenon (i.e., trivial side effect) of matter, leads to a debased view of human beings, who are seen as mere animals, machines, robots, or vehicles for genetic reproduction. The dignity of man is incompatible with philosophical materialism.
5. On a personal level, I feel that life simply has no meaning if "this is all there is." You live, work, suffer setbacks and occasional triumphs, then die and cease to exist. All the people who knew you will be dead soon, too, and you will be entirely forgotten. What difference did it make whether you lived or not? What's the point? For me personally, I do not see any way to find meaning in existence if human life is a mere accident and the cosmos has no higher purpose. I realize that other people are apparently able to find meaning on these terms, but I cannot. And I suspect I am not alone, as witness the "lives of quiet desperation" led by so many in our society today, despite the advantages of a very high standard of living, wide political freedom, and unlimited opportunities for leisure and recreation.
6. In studying history, I became aware of the very large contribution to human happiness, well-being, and moral advancement made by religion. Since I have read mostly about Western history, I focused on the effects of Christianity. Yes, I know about the Crusades, the Inquisition, Galileo's house arrest, the witch burnings, etc. - although all of these things have been exaggerated or misrepresented to some degree. But as bad as these events were, they need to be weighed in the balance with Christianity's positive contributions: greatly improved treatment of children, and an end to the practic of infanticide; greater rights for wives and widows, who were often helpless in the pagan world; much better treatment of slaves, and the eventual abolition of slavery worldwide (most abolitionist movements were religiously inspired); the end of the Roman "games" in which people were hacked to pieces in gladiatorial bouts or torn apart by wild beasts for the entertainment of the mob; a view of nature as stable and orderly, running according to natural laws enforced by God, which opened the door to modern science; the idea of an immortal, personal soul, which granted dignity to even the lowliest person; and on and on. (For more, see my essay, "Why I'm Not a Skeptic."
7. Finally, after being an extreme skeptic with regard to paranormal phenomena, I began to study the field and found that much of the evidence was unexpectedly strong. This includes evidence for life after death, such as near-death experiences and the better-documented cases of apparitions, deathbed visions, and mediumship. Of course, this topic is too large to go into here. Suffice it to say that I think there is sufficient empirical evidence to strongly suggest (though not conclusively prove) that our individual consciousness survives death. An afterlife does not necessitate the existence of God, but it certainly implies the existence of a supernatural realm. Various essays of mine on the paranormal can be read at www.michaelprescott.net/essays.htm .
So that's the Cliff's Notes version of my intellectual odyssey (which is, I must add, ongoing). There were other elements, as well. Reading about quantum physics convinced me that the "mystical" view of fundamental reality is probably closer to the truth than the mechanistic view. My observation of atheists indicated that many of them seem angry or at least perpetually disgruntled, while more religious or spiritually inclined people often seem calm and contented. (There are, of course, many exceptions to both rules.) I noticed that the worst mass murderers of the 20th Century were either atheists (Stalin, Mao) or pagan occultists (Hitler and his gang). I began to think that the ego is at the root of many human problems, and religion generally teaches us to overcome the ego and see life from a wider perspective. I could go on, but you get the idea. As my worldview changed, more and more things began to fall into place for me.
Again, I don't claim that this brief summary proves anything. I'm just setting out the broad outline of how my thinking changed. I imagine that many others have gone through a similar process - or will someday, even if they don't know it yet!