I really don't.
But then I used to not understand Christians.
How can they believe in something they can't see, or prove? And isn't the Bible just a collection of unprovable myths?
But, as the song goes, "I was blind, but now I see!"
I have been studying this so much over the past several years that now I don't know how anyone who gives it a little bit of thought, a little bit of research,can deny that God exists. I am of the conclusion that it takes more FAITH to NOT believe, than to believe.
Evolution: This is the biggest "weapon" of the atheist. Evolution is proof that we did not need a creator to exist. I ask, if Evolution is true, then why could it not be a tool by the almighty to which He brought us forth? So Evolution itself does not mean or prove that there is no God.
Time: another "weapon" of the atheist. Science shows us that the universe is billions of years old. The Bible tells us it is less than 10,000 years old. If the Bible is wrong about this, then how can I believe anything else? Gerald Schroeder, PhD from MIT has a great article The Age of the Universe that uses Einstein's Law of Relativity to show how both can be right, so again, we did not prove God does not exist.
The Atheist wants us to prove that God exists, but is unable to prove that God does not exist. You cannot prove a negative. If I say that no where in the universe does there exist a red stone with black stripes and blue spots, I can never prove it. Because I am unable to search the entire universe to prove it. To say that God does not exist is to say that you have searched the entire universe to know this is true, or that you are omnipotent which only God can be.
Now, I concede that I cannot prove God exists by taking Him by the hand and presenting Him to you. Which is pretty much the kind of proof that Atheists want. But I know He exists because I exist. The universe exists, Earth exists, so something created it. And the overwhelming complexity of life is too incredible to not be made by a creator. It exists, something created it.
Also, There is historical, archaeological and scientific evidences that reveal more and more what the Bible says is true. SO. If I believe there is a creator (because I exist) and I am learning that the Bible is true, then I must believe that God exists.
To deny it just takes too much faith.
There is a really great book by Lee Strobel called "A Case for the Creator" an another one called "A Case for Jesus". Lee Strobel was an atheist and is an investigational journalist. He set out to prove that God / Jesus do not and can not possible exist. But the more he researched and learned, the more obvious it was to him that God / Jesus do exist and are who they claim to be. I urge any doubters to read his book or watch his Case for Christ / Case for the Creator and then decide for yourself. It may be the most important thing you ever do.