“I was brought up to believe in God, therefore God exists” and “I was brought up to believe in God, therefore God does not exist” are fallacious arguments. Why? It goes without saying that the former sentence is not usually made use of, even though it could be heard in some kind of Red Neck chatter or at an informal Tea Party meeting. But as regards the latter, its use (or else the use of variations thereof) is attested by the millions.
But the fact is, the existence of wrong reasons to believe in x or -which is stronger- simply to believe in x does not imply the falsity or truth of x. You may have been forced to convert, or brainwashed so as to become a hardline atheist as a child; these claims do not defy the objective truth -whatever it may be- about God.
Let w be a possible world where God exists. Therein the proposition “God exists”, which we call henceforth E(g), is true. Therefore, the proposition “I believe that x”, whatever x is, clearly does not affect the fact that in w the proposition ”God exists” must be true according to the usually accepted semantic conventions; it includes x = “God does not exist” and x´ = “God exists” as much as x´´ = “Frank Zappa is dead”. That is to say, the proposition E(g) is independent of any given proposition of the form “I believe in x“.
That may be due to the neutral nature of belief statements as regards the truth-functionality of the thing which is subject of belief. The proposition “I believe in x” may be true or false, assuming that you could be sure of your, or someone’s, belief; but it does not affect the truth-functionality of x. It may be said that every adult white male (I’m kidding) agrees that unicorns never existed. But suppose something like an unicorn skeleton were to be found. It may be disputed whether the gefunden ‘unicorn’ could by rights be named “unicorn”, even if it were the case that it came to reflect, so to say, every property assigned to it by our actual myths. But it cannot be disputed that the discovery did not affect the truth-function of the proposition “unicorns never existed” -it was always false (if the name “unicorn” is agreed to be the proper designator for the thing) in w, the world in which the proposition is set to be true if the objective of the propostion exists (= the situation or fact that unicorns existed obtains).
This is so obvious people seem to ignore it.*
* What is not so obvious is whether one should or should not insert a comma between obvious and people.