Nicaea vs Nicaean

Nicaea is a noun but Nicaean is not a noun.

Word	Noun	Adjective	Verb	Adverb
Nicaea	Yes	No	No	No
Nicaean	No	Yes	No	No

Nicaea (noun)	Nicaean (noun)
the first ecumenical council in 325 which produced the wording of the Nicene Creed and condemned the heresy of Arianism
the seventh ecumenical council in 787 which refuted iconoclasm and regulated the veneration of holy images
an ancient city in Bithynia; founded in the 4th century BC and flourished under the Romans; the Nicene Creed was adopted there in 325

Nicaea (adjective)	Nicaean (adjective)
	of or relating to the ancient city of Nicaea in Asia Minor