“There is no such thing as a Christian child: only a child of Christian parents.”
“Let children learn about different faiths, let them notice their incompatibility, and let them draw their own conclusions about the consequences of that incompatibility. As for whether they are ‘valid,’ let them make up their own minds when they are old enough to do so.”
“Faith can be very very dangerous, and deliberately to implant it into the vulnerable mind of an innocent child is a grievous wrong.”
All quotes taken from "The God Delusion" by Richard Dawkins.
All quite simplistic and well known quotes, but this is really how I sum up my view on religion.
As a baby and a child you are a sponge, soaking up everything around you and learning everything from your immediate influences which are usually parents, family and family friends. As a child you are wired to accept without question what you are told to be true, this is necessary to enable you to grow up with a sense of danger etc but also lends itself to the ability to accept religion without question or disbelief.
This is why children who grow up in a non-religious environment usually grow up to be non-religious - they make up their own minds when they are older and capable of critical thought and reasoning. Children were brought up with religion have it 'hard wired' into them to an extent and as such find it more difficult to challenge and question.
Adults who have grown up with religion and then denounced it are generally more vehement in their arguments against religion, whilst adults who have grown up atheist tend to have a more "whatever, so long as it doesn't directly affect me/society" attitude.
As I said, this is just my very simplified view - and as such I think it would be more beneficial to debate with an adult who has 'chosen' religion as an adult, because they do not have the legacy of childhood belief.