The Chozeh of Lublin zy’a said that the mitzvah of relating the story of the exodus from Egypt has the power to remove all bad thoughts from a person’s head and transform them into pure and holy thoughts. He explains that when the Jews were in Egypt, they had thoughts of idol worship. (Chazal say in Shemos Rabbah 15:2 and 21:7 that some people actually served idols.) Hashem lifted them up from the 49th level of impurity and brought them up to an extreme level of holiness, thus removing those thoughts from their minds. When one relates this story to his children, he brings back this influence and allows their thoughts to become purified as well.
This explains the words of the Hagadah: ‘Anyone who speaks a lot about the exodus from Egypt is praiseworthy.’ This can be understood to be referring to a person’s neshama. Since he speaks a lot about yetzias Mitzrayim, he purifies his soul and cleanses his thoughts.
Annulling Bad Thoughts Through Emunah
The Rimanover Rebbe zy’a writes that that Pesach is the time of ultimate redemption, and contains so much holiness that one can reach a level of fully understanding Hashem’s essence. On this day, one can come to comprehend how Hashem’s greatness fills the entire world, which will cause a person to be ashamed of having improper thoughts, as he knows that Hashem is aware of everything he thinks.
There was a moment during the night of the exodus when the Jewish people reached a level of perfect emunah, and this influence remains for all generations. Even today, if a person is careful not to allow his thoughts to stray during the Seder night and he concentrates on the holiness and sanctity of the night, when that moment comes he can attain the same level of perfect emunah. Only our improper thoughts prevent us from feeling this.