Actually it can be...
Sound deductive reasoning:
Question 1: How did I get here?
Answer: I was conceived in my mother, and was formed in her belly, and then born into the world.
Question 2: How did my mother get here?
Answer: She got here the same way I did (i.e. was born from her mother).
Question 3: Ok, so how did all the people before her get here?
Answer: They most likely came into the world the same way I did, from their mothers, though I can't *prove* this with scientific evidence because I wasn't around when they walked on the earth the same as I do now, and thus did not witness their being born into the world. However, it is generally an accepted fact that each person all the way up the line of humans came from their mothers.
Question 4: Ok, so logic tells us that there had to be a *first* human (obviously, two, because we haven't met any humans who was conceived asexually) on the planet, who all the other humans descended from. So how did the first human(s) get here?
Answer: Well, here is your answer: God created him (and her).
There is your "proof" of existence in God. If you don't believe this, then most likely you believe a different theory: evolution. If you believe in evolution, then how do you explain how everything before humans got here? Logic tells us there has to be a first of everything, or else there wouldn't be a second, a third, and so on. So, as you can see, that is justifiable logical deduction to believe in the existence of a God.