I really know that for the levels of studying English language, we had always said that "for" is a coordinator. However, I would like to know what for serves in this sentence
- For God so loved the world.
Is it still a coordinator or any word class?
In your case 'for' is a conjunction.
According to Oxford English English Dictionary (https://en.oxforddictionaries.com/definition/for):
‘he felt guilty, for he knew that he bore a share of responsibility for Fanny's death’
As about the term 'coordinator' it's a synonym of 'conjunction'.
See in Oxford Living Dictionary:
2 Grammar A word used to connect clauses, sentences, or words of equal syntactic importance (e.g. and, or, for)
‘subordinate clauses can be connected with a coordinator’ (https://en.oxforddictionaries.com/definition/coordinator)