Is it obligatory to inform the husband of insulting the religion, and does the wife become unlawful by doing so?

You must offer sincere repentance to Allah for this reprehensible act and never return to it. As long as you repented before your iddah period ended, you are not forbidden to your husband. You are not required to inform him of what happened; rather, sincere repentance between you and Allah Almighty, fulfilling all its conditions, is sufficient for you.