It usually does little to undermine the foundation of beliefs that theism per se is grounded in.
And there is exactly where you are wrong.
It is exactly the foundation of the beliefs where most atheist disagrees with theist.
The fact that theists cannot know what say they know.
Not the details of their belief. Not the details of what ”god” is.
But HOW they (supposedly) know this.
That is how you view it, because you find it so straightforward to reject belief. You aren't motivated to "keep the faith". You don't put any hopes in prayers, have a community of believers that you've bonded with in support of your faith, engage in sacrificial acts, or engage in any of the activities intended to support the mindset. Unless you can get a religious person to question the foundations of belief in gods, not just the particular god that (s)he believes in, that person is likely to do what we all do in cases where some aspect of reality gets contradicted. We tinker with the edges of the belief that give us the most trouble. If you suddenly discover that your birth parents actually adopted you, you don't suddenly come to the conclusion that you had no birth parents. Your world is shaken, but you still believe that you had birth parents, even if the ones you believed in weren't the ones you thought they were. You can convince a Christian that his or her version of God is somewhat off in some respect, but that is rare. It isn't all that difficult to patch up the conceptual hole left behind, given a strong motivation to maintain faith. Usually, arguments are successful mainly with those who have already somewhat come around to the conclusion it leads to. The argument you make may help give them a little shove, but it isn't because you have shattered their world. Chances are, it already had a lot of cracks in it to begin with.