Is questioning the truth of Islam, and not resisting this matter within oneself, considered doubt or a whisper [from Satan]?

The mere occurrence of thoughts in the heart has no effect, and Satan deludes a person into believing that he has not shunned them. One must abandon these whispers and delusions, and be certain that one is upon Islam, and not allow Satan to corrupt one's religion and worldly life by indulging in these whispers.