1. Your definition of "psychosis" is wrong. Check the DSM, or any actual professional in the field, that is, what science says.
2. There is nothing "obvious" about the notion there is not a God, feel free to share the special insight you have superseding thousands of years of theology, philosophy, and science demonstrating clearly it is not "obvious".
3. Even if it were "obvious", the notion it is therefore "psychosis" is nonsense. Once can name innumerable instances of something being claimed to be "obvious" for which disregarding it is not "psychosis"--say, choosing to drink and drive despite the consequences. Say, buying overpriced products. Say, being Republican or Democrat, viewed from the other side.
In short, you are being irrational. In short, something is indeed wrong here with mental functioning. The person exhibiting this would be you.