We don't have synonyms for decretalist yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are decretist and decretals.
(n) :
(historical) A member of a 13th-century school of interpretation of canon law that emphasized the decretals (letters issued by the Popes on matters of church discipline) in preference to the Decretum Gratiani (1141), which their rivals, the decretists, favoured.
(n) :
(historical) A member of a 13th-century school of interpretation of canon law that emphasized the decretals (letters issued by the Popes on matters of church discipline) in preference to the Decretum Gratiani (1141), which their rivals, the decretists, favoured.