The Church teaches that The Guardian is a benevolent goddess who watches over the mortal realm and provides what aid she can. The Guardian reveres the light and that the Fel is a malevolent force seeking to consume the world. The old gods have very much been forgotten but old scripture refers to an age where there were many gods before a cataclysmic event resulted in their leaving the mortal plane and abandoned mortality. The Guardian now hears all the prayers and helps where she can, but The Guardian is neither omniscience nor omnipotent and is unable to answer every plea for help. This is why the Church teaches that the people of Solace should turn to the church for absolution and salvation.
Not all heretics are idealists, the Cult of the Shadow thinks that there was a war of Light and Dark and that the Dark won and that mortality wasn't abandoned but the gods of light lost. That the Fel is the Shadow Lord's army expanding across the mortal realm to expand the realm of darkness. The cult seeks salvation and immortality and pledges themselves to further the cause of the Fel, seeking to bring it into the walls of Solace and finish what he's started.