“A person might utter a word that he thinks harmless, which results in his falling the depth of seventy years into hellfire.”
Among the blasphemous kufur statement is for one to cuss Allah, any of the Prophets or an angel or to belittle the Religion of Islam. Some people when they are angry they cuss Allah or a Prophet or an angel. This is blasphemy kufur. They would have to utter the two Testifications of Faith to return to Islam.