I'm sure you can find a large number of books and websites stating why God exists and other sites stating he doesn't, many from intelligent, highly educated people, but all I will do here is say why I think he exists, besides the personal feelings and events of my life where God was involved.
As I work with other Zoologists, scientific and university people, we are use to working with a hypothesis of some type, so using one to 'prove' God, is I think possible. I know some people will immediately disagree and yes we can't make a formula or put God in a test tube, but we can look at it logically. The hypothesis - God exists.
At this very moment you, an extremely complex being are standing on a small, but extremely complex planet, full of extremely complex ecosystems, moving (at 400km/sec) through an extremely complex universe with extremely complex laws. Chances of this occurring purely by accident diminishes with every use of the words 'extremely complex'. Some people will say that there are billions, upon billions, upon billions of stars, so such a complex planet, complex ecosystems and our complex bodies could have 'just' occurred. Possibly true, but this still does not account for our complex universe and it's complex laws of science. Laws don't just happen or evolve by accident. Mathematics on which the whole universe is built on, is to me a beautiful example of a Master Mathematician! I have no problem with Evolution on our planet occurring, but I believe that a Master Evolutionist is involved.
We often want God and religion 'in a box' to fit our current ideas about life, but we still know so little about this universe or anywhere else. 'Where is God?', people often say. I've heard when the first Russian Cosmonaut Yuri Gagarin went into orbit around our little planet, he (or the Communist Party) said, 'I don't see God up here'. Do we really expect heaven to be up in the sky? Theoretical physicists have only fairly recently concluded that there are 10, 11, or 13 (or more) dimensions, instead of just the four. No one really knows how many there may be intersecting the very space you are in right now! Why couldn't one or more of these be the 'spiritual' world? I know it's a little out there, but remember we understand so little of this vast physical universe. As the Bible reads in Acts 17:28, 'For in him we live and move and have our being.'
You can chose to not believe in any creator God, but that will have it's own problems! You have to then have the courage to admit that you (and others) are purely an accident with no meaning, and your life doesn't count. Your just a freak of nature with no goal or destination. It make no difference what you do in life. Love, virtues or morals don't really exist. Any love you feel or anyone had for you, including your parents, is only evolution doing it's work. You can't tell anyone how to behave, because there are no lasting rules. Only what society dictates at the moment and what you think, your opinion. Opinions in human society change by the generation, so if Nazis had won the war, our laws and opinions on human rights (eg Jews) would be totally different! None are absolute or lasting. You honestly then cannot condemn a rapist, murderer or even paedophile for what they do. There are no absolute morals or any justice! A very sad universe, with no hope.
We humans as bad as we often are, do have morals (even the worst have some) and want justice. We may not be able to carry them all out, but we do treasure morals and justice. Even in our books and movies, we love a happy ending when the good guys win! As the Bible in Genesis 1:27 says, 'So God created man in his own image, in the image of God he created him; male and female he created them'. I do have hope, because I believe our universe has a creator God who is righteous and just.
I know this post on 'Why Believe In God', is very basic, it could go on for days, as there are so many other reasons. I think these are a few of the core ones and hopefully I'll be able to expand some others during future posts, including why Christianity (and what it really is) makes sense.