On August 25, 325, the Council of Nicaea ends with the adoption of the Nicene Creed establishing the doctrine of the Holy Trinity. From the article:
"The purpose of a creed is to provide a doctrinal statement of correct belief or orthodoxy. The creeds of Christianity have been drawn up at times of conflict about doctrine: acceptance or rejection of a creed served to distinguish believers and deniers of a particular doctrine or set of doctrines. For that reason a creed was called in Greek a σύμβολον (symbolon), a word that originally meant half of a broken object which, when placed together with the other half, verified the bearer's identity.[9] The Greek word passed through Latin symbolum into English "symbol", which only later took on the meaning of an outward sign of something.[10]
The Nicene Creed was adopted in the face of the Arian controversy, whose leader, Arius, was a member of the clergy of Alexandria. "Arius objected to Alexander's (the bishop of the time) apparent carelessness in blurring the distinction of nature between the Father and the Son by his emphasis on eternal generation".[11] Alexander accused Arius of denying the divinity of the Son and also of being too "Jewish" and "Greek" in his thought. Both Arius and Alexander rejected Gnosticism, Manichaeism and Sabellian formulae. The Nicene Creed was created as a result of the extensive adoption of the doctrine of Arius far outside Alexandria, in order to clarify the key tenets of the Christian faith.
The Nicene Creed of 325 explicitly affirms the co-essential divinity of the Son, applying to him the term "consubstantial". The 381 version speaks of the Holy Spirit as worshipped and glorified with the Father and the Son. The Athanasian Creed (not used in Eastern Christianity) describes in much greater detail the relationship between Father, Son and Holy Spirit. The Apostles' Creed makes no explicit statements about the divinity of the Son and the Holy Spirit, but, in the view of many who use it, the doctrine is implicit in it."