Some verbs which take a direct object need another noun, or an adjective, to explain exactly what happened. That noun or adjective is called a "complement" because it "completes" the action of the verb -- that is, it explains it fully. In this case, "appello" means "to name". "Deus appellavit aquas" tells us that God named the waters, but we still need to know what name He gave them.

Kirk's answer:

You're so very close: "part of the citizens took the riches...."

pars is nominative singular: it is your subject. divitias is accusative plural: it is what got taken. civium is genitive plural and is modifying the subject, pars. There's some fancy name for this use of the genitive, but how to translate it is pretty obvious.

It's at the beginning of the sentence for emphasis. It's in the sentence at all because it's the direct object.

Not necessarily. The genitive can be used with nouns, adjectives, and verbs.

With nouns, to quote Bennett's Latin Grammar, it further defines the meaning of the noun it modifies. That's the usage you're familiar with.

With adjectives, it does essentially the same thing. That's what's happening here. The genitives further define the meaning of "ignarus", explaining what, exactly, his soul was ignorant of.

With verbs it can sometimes be used as an object or an indirect object. Don't worry about that yet.

It's a three-termination adjective, Graecus, -a, -um. Therefore there are other possibilities.