The phrase is of course very well known:
Do you promise to say the truth, the whole truth, and nothing but the truth, so help me God?
I used to interpret “so help me God” as:
(a) God help me say the truth, i.e. God give me the moral strength to be truthful.
Recently I found that in the northeast of Portugal (in a mountainous area where men used to wear skirts and play bagpipes) people used to utter a similar phrase, “assim Deus me salve,” literally “so save me God,” as a guarantee that they were saying the truth or would keep a promise. Now God can help one be truthful but cannot save one be truthful. This plus context make it clear the Portuguese phrase means:
(b) May God save me if I’m saying the truth/I keep my promise (and not if I don’t).
The phrases are so similar that now I wonder whether the English one does not mean (a) at all but rather something like (b):
(c) May God help me if I’m fully truthful (and not if I’m not).
Online resources make me favour (c) only marginally. They say what the phrase is used for, but do not discuss the actual meaning:
Merriam-Webster: used to stress that a statement is serious and truthful.
Oxford Learners: Used to emphasize that one means what one is saying.
Longaman: used when making a serious promise, especially in a court of law.
So my questions are: (1) what is the correct interpretation of “so help me God?” and (2) would interpretation (a), even if it is not how people understand the phrase, be possible at all, i.e. would it be correct to say “(…) nothing but the truth, so help me God” to mean “(…) nothing but the truth, God help me keep my promise?”