A chassid once lamented to the Tzemach Tzedek that his faith was unstable and he often had doubts in his beliefs. The Tzemach Tzedek asked him: "What difference does it make to you?" The chassid responded with great emotion, "What do you mean, what difference does it make to me?!!"... The Rebbe then told him, "Your reaction [to my casual remark] indicates that your belief is intact."

Igros Kodesh of the Rebbe, vol. 17, p. 325