Meaning of "… is king"
Solution 1:
King is the title of the king, and titles can be used in the same manner as proper nouns; that is, some require articles and some do not.
If money is figuratively the king, then money is king.
The second example would more likely be "Christ is King", as Christ is indeed considered to be a king, and hence to have that title (and the title Lord, but using the two titles like that would be less common).