The names of cases are pretty arbitrary -- there is no law that a language has to have any particular cases.
Many inflected languages express functions through particular endings; for example, the actor of a given action, the thing/object the action affects, or the target of the action. As these are similar across languages, philologists are using the same names for these paradigms of endings, and call them cases, using the names from Latin grammar.
So the nominative indicates who executes an action, the accusative expresses the object or thing, and the dative the target. The genitive expresses possesion.
Now in English we don't really have cases (apart from pronouns), as function is expressed by word order or through prepositions.
The girl gives the dog a bone
The nominative is the default first noun group, and because give takes two objects, the dog is the 'dative' and 'a bone' is the accusative. But they're not marked. If you swap them around, you need to use a preposition:
The girl gives a bone to the dog
(You can also insert a preposition into the first example for emphasis: The girl gives to the dog a bone (and to the cat a piece of string))
So when designing your own language, you don't need to start off with cases a priori. Think about the functions you need to express, and then think of a way how to express them using morphology (word endings) or syntax (word order, prepositions/particles).
In your example, the son of god is given, one would call of god a genitive; German: "der Sohn Gottes", Latin: "filius dei" (note that English uses of, whereas German and Latin use endings). The son is nominative; as the whole example is in the passive voice and it is the subject. (It would be in the accusative if it was X gives the son of god).
So my advice is: if you're unclear about grammar, simply think of the functional relationships between the words. This is usually what cases express, and they are realised in different ways in different languages. If your conlang is not inflected, you use prepositions, otherwise endings. Whatever floats your boat... But don't get fixated on linguistic terminology from a specific language. Esperanto, for example, does not have a dative, but uses prepositions instead to express an indirect object (the dog in the example above), but the accusative for the direct object (la knabino donas al hundo oston, or la knabino donas oston al hundo)
As an example: give (as a conceptual act) has the following 'slots'
When you use a word expressing the concept of giving, these are the slots you need to fill (some are optional sometimes). How do you distinguish the slot fillers from each other? This is where your word endings or prepositions as markers come in.