only: a limiting word that narrows a statement down to one thing and shuts everything else out
This little word is a fence. It takes whatever follows it and marks that off as the single thing that counts, while ruling everything else out of the picture. Depending on what it fences in, English needs several different words to carry it: "only," "nothing but," "except," "however," "surely." But the underlying move is always the same one, the act of restricting.
Three shades grow out of that one move. When it narrows down an amount or a group, it means "only" or "nothing but" (only evil, only the priests, you only). When it carves out a single exception or qualification to something already said, it means "only that," "however," or "except" (do whatever you like, only do not touch the blood). And when it stands at the head of a clause to press a point home, it sharpens into "surely" or "certainly," insisting that this and nothing less is the case.
The shades shade into one another, which is why translators disagree about individual verses. What never changes is the basic gesture: this word draws a line and keeps the reader on one side of it.