A Muslim should always seek to control his tongue and not utter any words that imply blasphemy or atheism. He has to keep close to Almighty Allah at all times and never indulge in bad or blasphemous talk. The Prophet (peace and blessings be upon him) warned Muslims against slips of the tongue that may lead them to be punished in the Hereafter.
A person who unintentionally utters blasphemous words or statements should seriously repent and seek Allah’s forgiveness. Such grave words should not be taken lightly.
Sheikh Mohamed El-Moctar El-Shinqiti, Director of the Islamic Center of South Plains, Lubbock, Texas , states: Almighty Allah says: [Anyone who, after accepting Faith in Allah, utters Unbelief, – except under compulsion, his heart remaining firm in Faith – but such as open their breast to Unbelief, on them is Wrath from Allah, and theirs will be a dreadful Penalty.] (An-Nahl 16: 106)
Based on this Qur’anic verse, nobody would become a kafir (unbeliever) unless he or she intends to be. This does not mean that speaking or uttering words of disbelief is a minor issue; it is actually a major sin that should not be taken lightly or as a subject of humor and jesting. Any person who has spoken any word(s) of disbelief, must repent to Almighty Allah and seek forgiveness for the sin he or she has committed.