Let's pretend that God doesn't exist, but that I live as He does.
So here is what I suspect would happen. I choose to live my life as a faithful servant of God (who really does not exist). I choose to believe with all my heart that He does and that the Ten Commandments really are His rules for us to live by. I choose to fear judgement day every time I fail to act in accordance with those commandments and ask for forgiveness and trust in His mercy when I am sincere. And I choose to do that not only because I fear judgement, but because I love God and don't want to hurt Him. I screw up over and over every day and never stop believing in God's forgiveness and mercy and never stop seeking it.
So then I get old. I mean, really old. I am running along one day at the age of 95 (yep, still running!). I feel some pain in my chest and down my left arm. I get really dizzy and I realize, this is the end. I then fall down and die. From then on, I know nothing. God does not exist, but I don't care that I lived as though He did because I am dead. Gone. Kaput. I certainly don't feel duped or snowed… because I'm dead. My family buries me (or as I have requested, spread my ashes along the Boston Marathon course so at least I FINALLY can say I was on it. Even though I won't care because I'm dead and know nothing). My body rots and eventually disintegrates into dust. Only bones are left. Who cares? Won't bother me. I am dead. No soul, no nothing. I regret nothing. Because I am nothing. I simply, am not.
But God does exist. And suppose I live as though He does not or that it does not matter that He does.
And I'm running along at the age of 95 and fall down dead from a heart attack. And everything I chose to cast away as fairy tale or fantasy in regard to the existence of God suddenly matters more than anything ever did. It will determine where I spend eternity.
That's a HUGE problem. Not a risk I'm willing to take.
Live as though God exists. Because He does and it matters more than anything in life every will.