Proof.

Why hasn't God demonstrated his existence in "terms" which are incontrovertible?

God could present himself on the earth or in the heavens in all his glory for all to see and speak to us making his existence unquestionable. Carl Sagan (an atheist) asked why God does not just put a huge cross in the sky if he is real. Why doesn't he? It wouldn't be difficult for him. He created the heavens and the earth. Why does God not make himself obvious?

Love. Love is the answer. God is love (1 John 4:16). Love cannot be coerced. It cannot be forced. The only way for people to love God is if they have the opportunity to not love God. When I first met my wife, if I pointed a gun at her and said you will love me at this moment or I will shoot, she would have been unable to love me even to save herself. We cannot truly love God unless we have the opportunity to reject him. God loves us and wants us to love him. This has to be our choice by definition. In order for it to be our choice, God could not present the evidence for himself in incontrovertable terms.

Having said that, God has revealed himself to all people in atleast amounts that are enough to result in belief in him. Man has no excuse for rejecting him for the heavens declare the glory of God (Romans 1:20, Psalm 19:1).

The Wind...

God is invisible right? Atleast, I haven't seen him. So, how can we believe there is a God if we cannot visualize him? These are questions some of my friends bring up in discussions about whether there is a "higher power." They imply that if God is invisible that he cannot be real. I then ask if they can see the wind. This usually leads to a blank stare and a glazed look without verbal response. The fact is that we can know about the existence of persons or things through nonvisual evidence of that existence. In the winds case, we can see the evidence in the swaying of tree branches, the movement of clouds in a thunder storm, or the white caps that develop on the water. God has given evidence of himself in multiple ways. Creation declares the glory of God. Who can look at the stars and not wonder about the existence of a transcendent creator. God's Spirit moves in people in ways that glorify God-ways that make no sense outside of his existence. The Spirit is like the wind. We cannot see him, but we can see the people he moves as the wind nudges a tree branch. God's ultimate revelation is in his son Jesus and his story which is written throughout scripture. We do not have an excuse for disbelief as God has revealed himself to all people in significant ways.

The heavens declare the glory of God;
the skies proclaim the work of his hands.
Day after day they pour forth speech;
night after night they display knowledge.
There is no speech or language
where their voice is not heard.
Their voice goes out into all the earth,
their words to the ends of the world.
Psalm 19:1-4.

For since the creation of the world God's invisible qualities-his eternal power and divine nature-have been clearly seen, being understood from what has been made, so men are without excuse.
Romans 1:20.