I'm not sure disagreeing makes much sense if the question is "Is there a God?" What are you disagreeing to? That there isn't a god? Then you will be lying to your child and this is what the thread is about NOT doing.
If instead you mean, you must always disagree with whatever your child's opinion is, I don't disagree. You have to argue both sides of any philosophical question though, less you accidentally push your own beliefs on them, and this is extremely difficult to do correctly. Orson Scott Card's character Han Fei-Tzu is an excellent example of the type of teaching discipline I hold sacred. When someone makes an opinion about anything, you have to argue the other-side as well as you possibly can, and if your charge changes their mind, you have to switch sides and argue for that side. This requires an incredible amount of knowledge and intelligence which most parents are not going to be able to replicate.
Teaching in this matter will force the child to learn how to look at any problem from different viewpoints.
