It depends on its position in the sentence. Example 1: "إِلَهُكَ قوى" . It was written like this because it is "مبتدأ مرفوع بالضمة". Example 2 : " أحب إِلَهَكَ " ..Here it is " مفعول به منصوب بالفتحة " Example 3 : "أخذت من إِلَهِكَ الحكمة "..Here it is " اسم مجرور بالكسرة " It is all about "الاعراب"