Due to her being Atheist, he reincarnated her as a girl and forced her to become his prophet carrying a relic with a blessing coming from him. His goal is to make her and humanity acknowledge and praise his name as their Creator.
He goes into objects and dead bodies to force her to accept him as her creator, only to turn to taunts on her. And for her to have a undying hatred for him, and destroying anything he goes into to satisfy her hatred.
He is using her as an apostle a.k.a tool for his ambition to make humanity and Tanya accept him as their Creator.