Let’s assume that the notion of good remains the same whether you believe in God or not. Studies show that intrinsic motivation works better over the long term when compared to extrinsic motivation. If this is true, then using God to motivate good behavior is only good as long as people believe God is there to punish them should they go wrong. If belief ever fails, then, to the extent that believers have no intrinsic motivation, they will take advantage of the end to perceived punishment to do bad.
However, folks who are intrinsically motivated, carry their understanding that doing good is the right thing to do, no matter what their external circumstances are. They have figured out why doing good is good. They are not doing it for fear of punishment. They will never lose there desire to do good the way that believers will should their belief fail.
Now, I’m sure that many, perhaps most believers will eventually develop some intrinsic motivation to do good, even though the initial desire was motivated by punishment. But there will be a few who never develop intrinsic motivation, and once the fear of punishment is gone, they will see no reason not to do bad. This can not happen with folks who have always only had intrinsic motivation.
Now, people will not always have intrinsic motivation. They will have to learn it, and this will take a while. So children will be more likely to do bad things. There’s a learning curve about punishment, too, so it is doubtful that children in either system will be much different. However, to the extent that intrinsic motivate takes longer to learn something when compared to extrinsic motivation, children learning intrinsic motivation will do bad longer.
The other problem with extrinsic motivation is that it requires rules to teach people. You are not really teaching people to think for themselves. So, the instant something that is not covered by a rule shows up in the believer’s life, they will not be equipped to determine good from bad.
The intrinsically motivated person will always have a better algorithm for choosing good than the extrinsically motivated person. So, where it is not easy to decide between good and bad (or better and worse), non-believers will make better choices. Since the world is getting increasingly complex, and determining the right thing to do is getting much more difficult, training people to think for themselves, instead of relying on external rules, will make much better decisions.
To the extent that believers rely on rules instead of thinking things through, they are less well equipped for the modern world. This is why it is becoming increasingly dangerous to have people around who don’t think for themselves, or who think for themselves with a lower capability.
People who grow up in rules-based systems will always be at a disadvantage in determining right from wrong. Most will develop some intrinsic system for making those choices, but fewer will than people who grow up in non-rules based systems. To the extent that believers operate in rules-based systems more than those coming from non-rules based systems, the extrinsic learners will make poorer decisions when faced with morally ambiguous situations.
We are coming closer to the time where we can no longer afford to have people operating from rules instead of from their own choices. Rules-based religions and other rules-based organizations must either change or go away if humanity is to do well in the future.