God? No God? Who cares?
The point which the Humanist Society should have tried to get across with their ads is not that there's probably no God, but that it doesn't matter whether there's a God or not. But they had to go for the headline-grabber.
Point is, you should be trying to be a good person, which basically comes down to a lifelong Hippocratic Oath of "first do no harm". Every ethical position comes about naturally from that. And according to JC in the New Testament, it doesn't matter who we pray to - the good guys get to Heaven anyway, and the bad guys don't. The humanists and atheists are at an advantage here, because they can consider "first do no harm" without the laborious framework of how to do no harm which has built up historically around every religion - the utterly unholy Catholic doctrine of "every sperm is sacred", for example, which has no basis in the teachings of JC.
So it's the reverse of Pascal's Wager. Blaise Pascal reckoned that you should go to church regardless of whether you believed or not, because if the Christians were right then it'd get you to Heaven, and if the Christians were wrong then you'd not lost anything. But according to JC it's the other way around - you'll get to Heaven whether you follow a religion or not, so long as you behave well. And if there isn't a Heaven, you've still led a good life which has generally improved the world as a whole (even if only in a small way).
Besides which, there's a deeper point. Unless your god bases its decisions about an afterlife on the way you actually act, and the reasons you act that way, that god isn't worthy of worship. And if it does (cf. Christianity, Judaism and Islam), then it's handling admission to Hell, not Heaven.