The atheist says there is no God. But he can't prove it. And where the atheist really comes unstuck is when he can't explain how the entire universe came into existence out of nothing.
The agnostic concedes that there may be a God, but suggests that it's impossible to know for sure.
Is it possible to prove the existence of God?
Yes, it is. A preacher named Peter Grant has a good way of illustrating this.*
Suppose you decide to go to the office to see if your boss is in. You see his car in the car park. You ask his secretary if he's in. "Yes he is," she says. "I've just been talking to him." You see the light underneath his office door. As you get nearer, you hear what sounds like his voice talking to someone on the telephone. All this would be good grounds for concluding your boss is in his office.
But there's one more thing you could do. You could open the door and meet your boss face to face. At that point, the other things would still be valid, but would become secondary, because now you've met him face to face.
If you want to find out if God is real, you can consider stories of answered prayer; hear evidence of changed lives; read others' testimonies; study examples of fulfilled prophecy. All these things would be valid. But there's something more you can do. You can have something more personal than that.
Jesus said "unless one is born of water and the Spirit, he cannot enter the kingdom of God." "Born of water" is evidently a Hebrew expression for natural birth. In other words, if you want to enter the kingdom of God, it's not enough to be born naturally, you also need to have a spiritual birth. Jesus called it being "born again."
Don't laugh. Don't mock. Don't jeer. That would be allowing your prejudice to make the decision without putting it to the test.
Repent of your sins. (We're all sinners. We've all done things we shouldn't have done and not done things we should have done.)
Tell Jesus you're sorry for the things you've done. Tell Him if He's alive and He's who He says He is you will yield Him your life and follow Him. Ask Him into your life.
As He comes in He'll do a work in your life by His Spirit. You will enter into a personal relationship, and you will know that you know that you know that you know that God is real.
* Described by Lee Strobel in The Case for Faith. Grand Rapids, Michigan: Zondervan, 2000, p85.