I know what "so help me God" is used for, but what does it actually mean?
The phrase "so help me God," a popular component of grand and petit juror oath statutes, is actually an abbreviated form of the oath, "So may God help me at the judgment day if I speak true, but if I speak false, then may He withdraw His help from me." White, supra note 30, at 379-80 n.10.
This is similar to the Wiktionary entry:
The phrase implies that the speaker is willing to risk their chance of salvation upon their truthfulness.
So, it is (b)/(c), but with god's help being specifically towards what happens after the death of the speaker (eg, helping the speaker stay out of hell).
Some quote from a random religious website, to get an idea of the religious meaning of salvation (cf. your Portuguese word salve):
Salvation, or "being saved" means redemption from the power of sin. In practical terms, God's salvation is what we need to get to heaven or attain eternal life.
Christine Ammer, American Heritage Dictionary of Idioms, second edition (2013) has this brief but interesting entry for the expression:
so help me Also, so help me God. I swear that what I am saying is true, as in So help me, I haven't enough cash to pay for the tickets, or I wasn't there, so help me God. This idiom became a formal oath and is still used in courts of law for swearing in a witness (I swear to tell the truth, the whole truth, and nothing but the truth, so help me God). It was first recorded in 1508 as "So help me, our Lord."
Ammer's note about the earliest recorded occurrence seems to be incorrect, however, as an Early English Books Online search turns up two matches for "so help me god" from the 1480s. From William Caxtoon's printing of This Is the Table of the Historye of Reynart the Foxe (1481):
how bruyn ete the hony capitulo. .viij:
Bruyn eme I had supposed that ye had iaped therwyth / so help me god reynart nay / I shold not gladly iape with yow / thenne spacke the rede reynart is it thenne ernest that ye loue so wel the hony / I shal do late you haue so moche that ten of yow shold not ete it at one mele / myght I gete therwith your friendship /
And from Caxton's printing of Chaucer's Troilus and Criseyde (1483):
Lo he that is / as holy youres free
Hym recomaundyth / lowly to yowre grace
And sent yow thys letter / here by me
Auyse you on it / whan ye haue space
And of som̄e goodely answere / yow purchace
Or so help me god / pleynly for to seyne
He may not long lyue / in thys peyne
This same passage is rendered in The Riverside Chaucer, third edition (1987) as follows:
"Lo, he that is al holy youres free / Hym recomaundeth lowely to youre grace, / And sent yow this lettre here by me. / Avyseth yow on it, whan ye han space, / And of som goodly answere you purchace, / Or, helpe me God, so pleynly for to seyne, / He may nat longe lyven for his peyne."
Chaucer wrote Troilus and Criseyde in the early 1380s, which suggests that "help me God" as a way of asserting the truth of what one says may go back at least to the late fourteenth century; the exact form "so help me God" in the poem, however, may have originated in Caxton's 1483 printed version.
The Statutes Prohemium Iohannis Rastell (1527) identify two oaths that incorporate "so help me God" as an attestation of truth—the more easily understood of the two being the form for oaths of fealty for free men and for villains to recite:
Fealte
When a free man shall do fealte he shall hold his right hād vppon the boke and shall sey here you my lord that. I. A. B shall be to you feythfull and law full and shall bere you feyth of the tenementts that. I. clayme to hold af you and that. I. shall lawfully do you the seruice and costomes that. I. owght to do at termes assygnes as so help me god and all seintis / and a villayn shall hold his handis. vt supra. and shall sey / here you. & cetera. that. I. shalbe to you feythfull and shall bere you feyth of tenementis that. I. hold of you in villenage and. I. shalbe iustifiable to you of body and goodis as so helpe me. & cetera.
As for the 1508 instance of "so help me our Lord" mentioned by Ammer, that may be a reference to William Dunbar, The Tua Mariit Wemen and the Wedo. And Other Poems (1507[?]), which includes these lines:
I sall ye venome devoid with a vent large / And me assuage of ye swalme that suellit wes gret / My husband wes a hur mast{er} ye hugeast in erd / Tharfor i hait hī with my hert sa help me our lord / He is a ʒoung mā rytgh ʒaip bot noutgh in ʒouth flouris / For he is fadit full far et feblit of strenth / He wes as flurising fresche with in yis few ʒeris. / Bot he is falʒeid full far et fulʒeid in labour.
But this instance reached print (probably in Scotland) a quarter century after the first Caxton publication containing the phrase "so help me god" appeared.
As other answerers and commenters have noted, the literal sense of "so help me God" is roughly "with God's help," but I think that the substantive sense of it, from an early period, was (and is) "as God is my witness"—that is, "as I swear before God [to speak truly]."