I know that it is sinful for a mortal to claim to be God. However, if the person doing this has a mental illness and sincerely believes himself, is he still going to hell for blasphemy?
From the CCC (just google CCC to get there, or google CCC mortal sin for this specific page)
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."131
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
Edit: to clarify my position… only God knows what is in the hearts of men and women. Only God can judge the standing of our souls. I can’t tell you what would happen, although I strongly doubt conditions for mortal sin and Hell would be met if an individual was so truly psychologically disordered as to believe that they were God, and they were not making such a boast out of malice for the Living God,
If a person has a mental illness ,then I’m sure God would know about it…
I doubt it would be a sin,