Is the suspension of disbelief regarding the pronouncement of the two Shahadas effective without uttering it, or is utterance a prerequisite? And if the suspension necessitates repetition, how can one return to Islam?

The question is about excessive whispering (waswasah). The majority of scholars hold that whoever says, "I disbelieve in God if I do such-and-such," and then does it, does not become an infidel unless he harbored disbelief in his heart. They differed on whether an expiation for an oath (kaffarat yamin (definition)) is incumbent upon him. This applies even more so to one who did not utter it, because Allah has forgiven this Ummah for what their souls whisper, as long as the person does not speak it or act upon it. There is no sin from which repentance is not valid, for repentance is accepted from an apostate and others.