Is there any doubt that the disbeliever’s disbelief in his heart makes him a disbeliever?

Question:
If a person has doubt in his heart regarding the disbelief of the polytheists, but he does not verbalize it with his tongue, does that make him a disbeliever? And what is the difference between this and the whispers of the self?

Answer:
This is doubt. Doubt resides in the heart; its origin is in the heart. If a person wavers regarding the disbelievers, asking whether they are Muslims or disbelievers, then he becomes a disbeliever by that. If he verbalizes it, the matter is more severe. He says, “By Allah, I do not know; this one says such and such, and that one says such and such, and I do not know.” If he verbalizes it, it is more severe.