If I asked you what the electric field inside of a conducting sphere is, you might come up with an answer like "its zero." If I then followed-up by asking you how you came up with that answer since you have not been inside the sphere, you might come up with an explanation about how Gauss' Law can be used to show that the electric field on the conducting sphere is zero or that the net charge on a hollow conducting sphere is zero. My point is that there are various ways of knowing the truth of a matter that do not involve direct observation. A similar argument can be made about a claim that you have a spleen. Even though, I have not seen your spleen, I will be willing to grant you the benefit of the doubt if your life depended on my accepting the claim that you have a spleen since most healthy living humans have been found to have spleens. In a rare case, a genetic mutation might cause someone to be born without a spleen and yet survive without the spleen. No mortal man can see God. If you still want to know whether God exists even though we cannot see him, there are ways of finding out the truth of the matter. If you set out to know the truth about the existence of God by devoting your energies to finding reasons for disbelieving in the existence of God, you might miss the boat by a mile. There are certain things about God that are spiritually discerned. You cannot know them without first walking with him (by that I mean granting that he exists) and seeking to know him. I will summarize some of the reasons for my belief in the existence of God below.
1. The human being: The different parts of the human being must have been assembled at once. Consider a Boeing 747 aircraft. It has several parts that are essential to its operation. If any one of those parts malfunctions, the aircraft is doomed on its next flight. Likewise, if a man is missing any of the organs that are essential to its survival, that man has a very small chance of surviving more than a few days without that essential organ. If man evolved from single celled organisms into the complex beings that we are over billions of years, that would mean that at some point man had to evolve a heart, lungs, reproductive organs, an eye, a nose, etc. Nature does not owe man anything and has no business waiting for man to evolve all these organs. The chances of single celled organisms surviving long enough to evolve all these organs and eventually becoming the complex creature that is man are close to zero.
2. The perfect motion of the stars and conditions that make the earth habitable for humans: If the universe was governed by randomness, many planets should have collided by now, but what we find instead are planets whose motions are governed by universal laws. Again, if the earth was too hot or too cold for humans, we would have gone extinct long ago. It is by divine providence that we have plants whose seeds and leaves are herbs for the healing of many sicknesses and organisms whose products or by-products have helped prolong life.
3. The ability to think and act rationally to any degree worth talking about: Human beings are the only creatures endowed with this ability. If this ability could be evolved, you would have to explain to me how other creatures who evolved alongside humans have not developed the ability to build houses, write books, learn different languages, etc.
4. Fruits, herbs, light, day and night, speech: It could only have been by divine providence that we have the sun to give us light by day, and the moon to give us light by night. Again, nature does not owe us much of the benefits we enjoy on this planet. We have fruits for our enjoyment, herbs for our healing, speech to allow us to communicate, the sun to provide light, eyes to make us see, ears to make us hear. Will you open your eyes to see God's providence in the earth and your ears wide enough to hear his words?
5. The fulfilment of ancient bible prophecies (He declares the end from the beginning). There are many bible prophecies that can be proven to have been fulfilled, but I will stick to the two listed below to keep things simple.
Only an all-knowing God could have predicted the exile of the Jews and the coming of Christ, who was born in Bethlehem at the time prophesied by Daniel. The dead sea scrolls and other archeological findings such as the remains of the temples in Jerusalem (Solomon's temple and latter temple) prove that there were Jews in Jerusalem three thousand years ago who worshipped God in that temple as mentioned in the bible. The dead sea scrolls are old enough to support the claim that the old testament prophecies did indeed precede the coming of the Messiah. The prophecies were not written after the event.