A "meta-argument" against all future arguments for God's existence ?

[Angrboda]

I ask the same question to theists:
With the presents of god, how do you trust your naturally evolved brain to be a reliable tool to investigate reality, and not a delusion?

Because, assuming theism, God intended to create human beings who would actively and freely seek to know Him. This entails endowing them with brains capable of investigating reality and acquire knowledge.

Assuming is not knowing. You can assume that is what God did but you don't know that's what he did as he may have had other ideas. In fact, we know he left certain backdoors in our reasoning by which it could be corrupted so as to allow the hardening of hearts. And Satan rules on earth, explicitly, assuming God wouldn't allow you to be deceived gets you jack squat.

Everything is a mechanism based on what we have observed so far.
At the foundation, the mechanism are based on the properties of a few fundamental particles.
It is also easy to explain why nature is not capable of making watches and it has nothing to do with complexity.

Going back to the foundation won't help you here: the fundamental particles were clearly fine-tuned all along to yield our universe, if that doesn't point one to a designer then it's difficult to see if anything would be enough for them. besides that, nature is not capable of anything, because it's not a personal agent. 

You have no evidence that they were fine-tuned. Random chance explains them as parsimoniously as design.