Will a person who curses the religion unintentionally and utters a word of disbelief without intending it be held accountable, and does the Hadith "My Ummah is pardoned for their mistakes and forgetfulness" apply to him?

If expressions are uttered in a state of bewilderment and absence of mind, there is no blame; as Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended." However, if that occurs due to negligence and without thought, then sincere repentance is obligatory; because reviling the religion of Islam is disbelief that necessitates uttering the two testimonies of faith and repentance, and accusing a Muslim of disbelief is one of the gravest sins.