If God did not exist, I would die. That is twofold: firstly if God did not exist, then I would not exist - I would not have been brought into being or sustained in every molecule, sub-atomic particle and breath. And secondly if I were convinced that God did not exist and it turned out that I still existed in some way then I would be a nihilist and die. Without God there is no reason to live: for love of others - but whence love of others? For love of life - but what life? For love of self - but what self? For from him and through him and to him are all things.
Nietzsche reasoned that if (since) man did not act as if God exists, then God was (effectively) dead. But if man acts as if God does not exist, then it is not because God is dead, but that man is dead. It has been so since Genesis 3. Man acted as if God did not exist as God, and died. (Further, Nietzsche's nice idea that this 'death of God' creates unimagined possibilities for those strong enough to meet them is strikingly similar to the devil's idea: 'You will not surely die... You will be like God...'!)
So then, since I am convinced that were God not to exist, I would cease to exist or die, and that if I were convinced otherwise I would be dead, in what ways do I act as if God did not exist? When do I act as if he doesn't exist, good, sovereign and righteous? Because whenever I do, I'm a dead man walking.
And you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience - among whom we all once lived in the passions of our flesh, carrying out the desires of the body and the mind, and were by nature children of wrath, like the rest of mankind. But God, being rich in mercy, because of the great love with which he loved us, even when we were dead in our trespasses, made us alive together with Christ ... For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them. [Eph 2:1-5,10]
Update: Ref21 ponders "Could it be that Nietzsche is actually one of the most influential evangelical thinkers of all time?" in a blog post.