How Can You Be Sure Christianity/Islam/(insert others here) Is Not The One True Religion?

I have been asked this on numerous occasions. It is clear to me that there is not one universal god, or one universal set of gods, for all of humanity.

The crux of this argument is the sheer number of different god-centric belief systems, that have been developed in parallel by cultures which for many centuries had no contact with each other. The Aboriginal Australians developed the concept of "dream time", the Native Americans came up with a complicated system of spirits and associated beliefs, the ancient societies of Greece, Italy and Egypt had a plethora of gods, and so on.

Are any of these concepts of "god(s)" mutually compatible. Well, some might be but many are totally beyond combination. The multi-god religion of the Romans is totally incompatible with the one-god teaching of Abrahamic religion. The spiritual unity of the natural world in many religions also clashes harshly with the Old Testament's insistence of the superiority of man over the animals.

Surely if there was only one god, or group of gods, then all religions would have a common focus, even if they were not all identical. Surely the natives of Australia should be as capable as any as following Jesus?

Or, just perhaps, the fact is that the inhabitants of the middle-East around 2000 years ago (and their favourite imaginary friends) didn't know about Australia at the time. It posed no threat to them and did not need to be controlled by the same rules - enforced through the promise of eternity in paradise, or hell.

The sheer number of mutually exclusive belief systems that claim to be the only way to salvation should make the followers of any of them question their beliefs. The strength of their belief is equalled by other believers who believe in a different system that also offers the only way to eternity in paradise.

It wouldn't be half as bad if they just got on with life, but how many times in history has one of these belief systems launched an attack on another? How many millions have died? What a waste of human life and potential!

But the worst is yet to come, these people who stand by an organisation that has committed genocide, murder, robbery and countless other crimes on people who have the audacity to believe in a different fairytale then tell me, as an atheist, that I have no morals.

Well thankfully I live in a society where it is generally acceptable for people of intellect, rather than men in robes, to come up a set of rules to live our lives by. Although that doesn't stop the 'robies' trying to interfere with anything they can get their hands on.