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.