"Penitent One. Meet me in the chapel where Guilt was given shape. Where you were anointed the Penitent One and where you knelt for the first time."
The Mea Culpa Chapel is a small area added to the game through the Wounds of Eventide DLC, featuring the place in which the Penitent One obtained the Mea Culpa sword prior the events of the game.
Accessing it requires to follow the steps needed to obtain the True Ending.
The doors will open after the Penitent One confronts Esdras with the Incomplete Scapular, obtained through Perpetva.