Does my fasting become void due to the intrusive thoughts and ideas I had about the existence of Allah Almighty, even though I repented, sought forgiveness, and returned to my fast?

The treatment for whispers (waswas) is to completely disregard them and occupy the heart with what is beneficial. Passing thoughts (khatir) do not affect the soundness of faith or fasting unless one speaks them or acts upon them, according to the hadith: "Indeed, Allah has forgiven my Ummah for whatever occurs to themselves as long as they do not speak of it or act upon it." One must seek refuge with Allah from Satan, engage frequently in the remembrance of Allah, and avoid isolation.