The main verb here is "pass'd", and what you're missing is the now rather archaic (and in this case divided by an intervening phrase) usage of the phrase "not one but". I can't find a good dictionary reference for this, but, in the English of Keats's time, "not one but did X" meant (and still means, but so rarely used now as to be almost obsolete) "everyone did X".
I'd guess, without proof, that this was probably originally a holdover from the French "ne ... que", which is similar, word by word, to "not ... but", and which functions as a sort of double negation to mean "only".
You can find this usage in various other older pieces of literature, including 20th-century ones, several of which have previously been asked about on Stack Exchange:
One by one he subdued his father's trees
By riding them down over and over again
Until he took the stiffness out of them,
And not one but hung limp, not one was left
For him to conquer.
-- Robert Frost, "Birches" (1915), previously asked about on Literature SE
Peruse them well:
Not one of those but had a noble father.
-- William Shakespeare, All's Well That Ends Well (1623), previously mentioned on Literature SE
There were nearly twenty-five million inhabited planets in the Galaxy then, and not one but owed allegiance to the Empire whose seat was on Trantor.
-- Isaac Asimov, Foundation (1942), previously asked about on English Language & Usage SE
So your Keats passage could be rephrased in more modern language as:
The shepherds always believed
That every fleecy lamb which thus departed
From the white flock would pass unharmed
By angry wolf or preying leopard,
Until it came to some untrodden plains
Where Pan's herds fed: everyone gained greatly
If he lost a lamb like this.