Is merely uttering the two Shahadas sufficient for repentance from apostasy, and must they be recited publicly?

Mere whisperings (waswasah) do not cause one to leave the fold of Islam, unless the person intentionally commits acts or says words of disbelief. What is done unintentionally does not make one disbelieve. Therefore, turn away from these thoughts, continue with prayer and study, and occupy your time with fruitful work or permissible recreation.