Does a Muslim become impure due to obsessive-compulsive disorder (OCD) which causes him to mock the religion, such as when he believes that the sun rising without rays is a lie?

One is not held accountable for inner thoughts unless spoken or acted upon. This is by God's mercy towards His servant. We ask God to cure you of obsessive thoughts, and we advise you to persist in supplication and humility, and to distract yourself from these obsessive thoughts. We also recommend that you consult a trustworthy psychiatrist.