In the sentence "The purpose of life is to serve God," is 'God' a direct object or an indirect object?

I'm writing an essay for my Spanish class about the beliefs of the Abrahamic religions, and I'm not sure whether, in this case, I whould write "La vida es para servir a Dios", "es para servirle a Dios" or "es para servir Dios." I'm using "servirle a Dios," with the reasoning that one is serving *for* God, not serving God in the same sense that one might serve chicken at dinner time. However, I did a search of all three possible ways of saying "to serve God" in Spanish on the internet and found that all three forms are used. Are they all correct, or is one preferred?

If it is an indirect object, why can't we say "The purpose of life is to serve to God" in English?

From 'The New Penguin English Dictionary':

'Serve' has 14 definitions listed. One is actually "to give due honour and obedience to (God)". If you say serve to God, it sounds like you're using definition '3', but mistakenly omitting an object - "to serve (beer) to God".

So, in English, in the phrase 'serve God', 'God' is the direct object, and in that context, 'serve' has a particular meaning.

In Spanish the thing receiving the service is always the indirect object. Le sirvo a Dios. The direct object is implicit. If God came to Thanksgiving dinner, for example, You'd say Le sirvo pavo a Dios.