Among those, I sorted out the fifty most pious (7)
I believe the answer is:
holiest
'most pious' is the definition.
(I've seen this before)
'among those i sorted out the fifty' is the wordplay.
'among' indicates putting letters inside (I've seen this in other clues).
'sorted out' indicates anagramming the letters.
'the fifty' becomes 'l' (roman numeral).
'those'+'i'='thosei'
'thosei' is an anagram of 'hoiest'.
'hoiest' enclosing 'l' is 'HOLIEST'.
(Other definitions for holiest that I've seen before include "Most pious" , "most befitting to worship" , "Most divine" , "superlatively untouchable" , "Most resembling sacred work" .)