I think you're just supposed to be, lore-wise, oath bound to any deity you wish, and make your own head canon as to why your character is so devoted to even evil deities. I suppose similar to the cleric Shadowheart from Baldur's Gate 3. She worships Shar, the goddess of darkness and loss, who is neutral evil, yet Shadowheart can still cast holy spells, potentially the holy power you wield in this game is rather tied to the strength of faith itself, rather than the power of said God. It states on the wiki so.