"Богу" is Dative case of "Бог" - "God". Examples - Благодаря Богу (кому?) мы встретились. - Thanks to God we have met. Молиться Богу - to pray to God. The word is masculine and is declined so: Nominative - Бог Genitive - Бога Dative - Богу Accusative - Бога Instrumental - Богом Prepositional - Боге