Is there a word for someone who devotes all his time and energy to something he considers more important than himself? Something like a clergy, priest, or monk, but without the religion.

Workaholic seems to be used as such, sometimes, but its origins seems to be more about a compulsive behaviour.

In case it helps somehow, priesthood is translate to portuguese as "sacerdócio" (with "sacerdote" meaning "priest", "monk"), which is used sometimes in the sense that I look for: something that you do with your life (usually helping others) that you believe to be very important. This could apply to doctors and teachers moving out to a poor area and working for peanuts, or someone choosing to become a fireman even aware that he is not going to be well paid, for example.

EDIT: some synonyms for "sacerdócio" can be translated as "mission", "responsibility", "incumbency". But I was wondering if there is a more specific word.