My first thought was – wow.  All Law and no Gospel.

So if a person who lives as though there is no God saw this bumper sticker and said, "Oh no, what if there is a God?" and starts cleaning up his moral act – does that mean the person is going to heaven?  Not without the forgiveness of sins through Jesus Christ.  Even if the person from that time forward lived a perfect life (which is impossible in actuality), that wouldn't make up for everything wrong he had done previously.

On one of the podcasts that I listen to, someone once said something to the effect that if Satan took over a city, he wouldn't make it a city where there's no law and order and everyone is engaging in crime and there's mayhem and evil everywhere.  No, it would be a city where everyone is a respectable, law-abiding citizen, the children are all well behaved, where all the people are moral and go to churches every Sunday where Christ is not preached.

There needs to be repentance AND the forgiveness of sins.  Otherwise all you wind up with is the confusion of Law and Gospel, a kind of "clean up your act or you're toast!" sort of legalism that saves no one.