David starts two Psalms with "The fool has said in his heart there is no God" (Psalms 14 & 53).
On the first of April, people will be trying to make fools out of others. That's just a bit of fun. But when one makes a fool of oneself, by disbelief in God, nobody laughs. Why is a person a fool not to believe in God?
The creation leads a sensible person to acknowledge God's existence. The heavens are telling the glory of God, and the sky is declaring the work of his hands" (Psalms 19:1).
¶ “18For God’s anger is revealed from heaven against everyone’s ungodliness and unrighteousness —against all who supress the truth in wrongdoing. 19For what may be known of God is manifest in them, for God has shown it to them. 20Ever since the creation of the world, God’s invisible things, his eternal power and divinity, have been clearly seen by God’s works. So the ungodly have no excuse.” (Romans 1:18-20).
We may not be able to know or to prove the certainty of God's existence if we look only at the natural world. But the hypothesis at least cannot be denied, except foolishly. There is exquisite design and order in the created world, all the more remarkable when we observe it emerging from chaos.
In the face of this, it is foolish to say "There is no God."
God has revealed himself to mankind. God's truth was placed in the world. Paul asserts that, since the creation of the world, "they knew God" (Romans 1:21). Even pagan peoples retain some of that revelation in recognisable form. Then of course there is the Holy Bible which reveals God and his purpose for mankind.
God revealed himself in a special way to one nation Israel. To them God gave "the glory and the covenants" (Romans 9:4) in a special dispensation that is one of the best documented and detailed series of events in ancient history.
We are not talking here merely about a book, the Old Testament, although its credentials are unassailable. But we are talking about a race of people (the Jews) and a city (Jerusalem) which are the tangible outcome of God's special revelation to, and special relationship with the original Israelites.
In an even more special manner, God revealed himself in Jesus of Nazareth. The testimony about this person's existence, and about his crucifixion and resurrection from the dead, is in the form of written eye-witness documents.
The story of Jesus Christ is a matter of fact, not of superstition. "The Word became flesh and dwelt among us, and we beheld his glory" is one eye-witness's testimony (1John 1:1-4, John 1:1,14).
There are no reasonable grounds for rejecting such evidence. None but “a fool” disbelieves in God, and in his scheme of redemption for life everlasting through his Son.
You cannot sensibly say, “There is no God.” because you would have to know all things and be aware of everything that exists. Can human beings say, “We know and have discovered all things”? If not, then they are in no position to intelligently say, “There is no God”.
If God "dwells in unapproachable light which no [mortal] has seen or can see" (1Timothy 6:16), then we cannot discover God directly.
Of course we can determine the existence of God in other ways, as we have discussed. It is thus foolish to say God doesn't exist on the grounds that current scientific method hasn't found him.