God is meaningless
Relative Philosophy
Some philosophers think that religious language doesn't mean anything at all, and therefore that there's no point in asking whether God exists.
They would say that a sentence like "In the beginning God created the heavens and the earth" is neither true or false, it's meaningless; in the same way that "colourless green ideas sleep furiously" is meaningless.
Logical Positivism, or Verificationism
Logical Positivists argued that a sentence was meaningless if it wasn't either true or false, and they said that a sentence would only be true or false if
If it could be tested by an experiment,
OR
If it was true by definition
A more accurate version of this idea can be found here
Since you couldn't verify the existence of God by any sort of "sense experience", and it wasn't true by definition (eg in the way "a triangle has 3 sides" is true), the logical positivists argued that it was pointless asking the question since it could not be answered true or false.
These particular philosophers didn't only say that religious talk was meaningless, they thought that much of philosophical discussion, metaphysics for example, was meaningless too. This philosophical theory is no longer popular, and attention has returned to the issues of what "God" means and whether "God" exists.
Note for philosophers
This is how one prominent philosopher put it:
We say that a sentence is factually significant to any given person, if and only if, he knows how to verify the proposition which it purports to express-that is, if he knows what observations would lead him, under certain conditions, to accept the proposition as being true, or reject is as being false.A. J. Ayer
Ayer actually preferred a weaker version of the theory, because since no empirical proof could be totally conclusive, almost every statement about the world would have to be regarded as meaningless.
"A proposition is said to be verifiable, in the strong sense of the term, if, and only if, its truth could be conclusively established in experience. But it is verifiable, in the weak sense, if it is possible for experience to render it probable."
And this led Ayer to dispose of the God question rather brusquely:
"...There can be no way of proving that the existence of a god...is even probable.
"For if the existence of such a god were probable, then the proposition that he existed would be an empirical hypothesis. And in that case it would be possible to deduce from it, and other empirical hypotheses, certain experiential propositions which were not deducible from those other hypotheses alone.
"But in fact this is not possible...For to say that "God Exists" is to make a metaphysical utterance which cannot be either true or false."