Institution roughly two millennia old (6)
I believe the answer is:
papacy
(Other definitions for papacy that I've seen before include "Vatican leader's office" , "Job in Rome" , "clerical office" , "RC leader's office" , "religious role" .)
I believe the answer is:
papacy
(Other definitions for papacy that I've seen before include "Vatican leader's office" , "Job in Rome" , "clerical office" , "RC leader's office" , "religious role" .)