Christianity's ___ Creed (6)
I believe the answer is:
nicene
(Other definitions for nicene that I've seen before include "Name given to main Christian creed" , "This Creed is a formal statement of Christian belief from 4th century" , "Christian Creed adopted in 325 a.d." , "This Creed is a formal statement of Christian belief 325 a.d." , "Like creed" .)