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By the way, you're seeing the effect of air, not air. Air makes the bubble, but you can't see what fills the bubble.
That's because "air" is a concoction of a variety of transparant gases...however, if you prefer, there are many, many gases that are coloured. Most of them are rather dangerous though, and I wouldn't recomend you set out to see for yourself

We also have a variety of methods to detect things such as air pressure, or composition...suffice to say, there are numerous ways we can detect it that establish it is, without a shadow of a doubt, there. Unlike god.
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You're not seeing gravity, you're seeing what gravity does to a book. =D You can't see gravity itself.
Well, again, "see" is perhaps not the correct term...however, again, there are several methods we have of detecting and measuring the force of gravity. Rather like air...we know it's there, because we can detect it, we can measure it, and we can observe it's effects in conditions that, when duplicated, continue to produce the same result.
None of these methods can be applied to god...god exists only in the minds of the people who follow him - there is no evidence of his presense, his influence, or his hand in anything going on in the world. For all intents and purposes, there is absolutely nothing to make your god any different from the gods worshipped by the ancient Greeks...the only difference being they had more imaginative names.
Like your god, they could neither be measured or observed and, like your god, belief in them gradually diminished. Given time, I'm sure Christianity will be replaced by something new, with go-faster stripes, and people will look back and wonder how anyone believed such rubbish...that's assuming society doesn't simply become athiest - we wonder that already.