Is a person's saying to themselves silently, "I will apostatize," considered an act of disbelief (kufr), and what are the consequences if it is?

If a Muslim resolves to disbelieve in the future, then he disbelieves immediately, as stated by Imam Ar-Ramli, Az-Zarkashi, and At-Tarabulsi. It is obligatory for anyone who falls into this to renew his Islam and utter the two testimonies of faith (Shahadatayn). However, if what he experienced was merely a whisper (waswas), then he should seek refuge in Allah and not indulge in overthinking.