In law the "Faith" is presupposed as a prior act inherent to the person, it can be good or bad said act, in some countries public or private authority figures such as Notaries, give "faith" that some relevant fact from the juridical view is legal, or can be taken for granted. Since the figure of notary is not in itself easy to obtain, extensive studies and a license to practice the career are required, so finally depending on the context the phrase "A fe mía!" is a way of expressing the personal opinion. Some synonyms, words or similar expressions can be in my personal opinion, I think personally, according to my perception it seems to me
