Sefeikos in Emuna:
It is natural for the animal soul to challenge a person with Sfeikos in Emuna. These seemingly innocent inetlectual questions are in truth the working of Sitra Achra, as the soul of a Jew remains constantly truthful to Hashem and never has any doubts. The Siotra Acchra was given permission to confuse the person and challenge him in his faith in order increase his reward, simialr to the parable of the harlot hired by the kinbg to challenge the crown prince.
 Tanya chapter 29