How do you prove that there is a God to a friend that insist that there isn't a God? Can you teach me how? Thanks.
The first thing that you need to ask yourself and your friend is: would any proof of God's existence be adequate to your friend? Some people have already made up their minds. You can give them the most iron-clad case for the existence of God, yet they will always demand more. Often this is because acceptance of God means having to alter your personal life. I once studied with a young man who bounced from one sex partner to the next without much thought. As I proved to him that the Bible said his lifestyle was wrong and harmful, he would respond that he wasn't certain whether the Bible was true. As I poked at this, it was apparent that deep down inside he did believe the Bible was God's word, but so long as he could continue insisting that he wasn't certain, he felt justified in living a life contrary to the teachings of the Bible. No amount of proof would be acceptable to him because to accept the proof would mean that he would have give up his multiple sex partners.
The second thing to ask is "What evidence does your friend have that God does not exist?" Too often as Christians we find ourselves on the defensive, but we don't have to be. An answer to this question will give you some idea which way you need to approach the topic.
Third, start with the approach suggested by the apostle Paul. "For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known of God is manifest in them, for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse" (Romans 1:18-20). Paul states there is evidence within the world around us of not only the existence of God but also evidence of His character -- evidence that can be understood by examining what God has made. A good starting point is the orderliness of this world. Science tells us that entropy (chaos) increases when intelligence is not applied to a system, so how did this world become so sophisticated? Evolution, by its very nature, cannot explain this without claim this law of science to be wrong.
Another approach is to begin looking at the many irreducible systems within nature. There are many complex systems that require multiple parts working in harmony to accomplish a goal. For example, there are numerous chemical reactions required to cause blood to clot. Missing any step would mean that a creature would bleed to death from a wound. So how could these multiple step systems gradually evolve?
An honest look at our world leads a person to conclude that there must have been intelligence behind the design of our world. To acknowledge that such intelligence exists is the first step toward acknowledging the existence of God.