What is the ruling on saying "God does not exist" in a moment of anger and despair, while feeling that one has left Islam due to verses such as "Indeed, Allah does not forgive association with Him" and "And if you ask them, they will surely say, 'We were only conversing and playing'," and how can one expiate for that?

Whoever denies the existence of Allah is an apostate and must repent and return to Islam.

One must be extremely cautious of anger and despair of Allah's mercy. Anger gathers all evil, and its cure lies in avoiding its causes, seeking refuge in Allah from Satan, changing one's posture, and performing ablution.

Despair and hopelessness of Allah's mercy are characteristics of disbelievers and those who have gone astray. A believer must not despair no matter how difficult matters become.

One must be diligent in prayer because it contains healing and assistance in solving problems.