Quotes Showing the Trinity was Discussed at the Council of Nicea

With Scripture alone, and no other traditional or doctrinal presumptions, one cannot decisively end up with the orthodox understanding of the Trinity as defined in the Council of Nicea. One, in fact, can come up with any number of explanations of how God the Father, the Lord Jesus Christ and the Holy Spirit coexist and relate. However, beginning from the Council’s decisive definition of the Trinity, one can return to Scripture and see how this dogma is clearly illustrated in the truths of Scripture.
[Quoted in "Before You Object" by: By Marcus C. Grodi http://www.chnetwork.org/journals/mary/mary11.htm]

What we have undergone -- persecutions, afflictions, imperial threats, cruelty from officials, and whatever other trial at the hands of heretics -- we have put up with for the sake of the gospel faith established by the 318 fathers at Nicaea in Bithynia. You, we and all who are not bent on subverting the word of the true faith should give this creed our approval. It is the most ancient and is consistent with our baptism. It tells us how to believe in the name of the Father and of the Son and of the holy Spirit: believing also, of course, that the Father, the Son and the holy Spirit have a single Godhead and power and substance, a dignity deserving the same honour and a co-eternal sovereignty, in three most perfect hypostases, or three perfect persons. So there is no place for Sabellius's diseased theory in which the hypostases are confused and thus their proper characteristics destroyed. Nor may the blasphemy of Eunomians and Arians and Pneumatomachi prevail, with its division of substance or of nature or of Godhead, and its introduction of some nature which was produced subsequently, or was created, or was of a different substance, into the uncreated and consubstantial and co-eternal Trinity.
[Quoted from: A letter of the bishops gathered in Constantinople" http://www.piar.hu/councils/ecum02.htm]

First Council at Nicaea (#1) 325 AD

The First Council of Nicaea (325), the first ecumenical council, devoted itself to the problem of the Trinity, in an attempt to settle the controversy raised by Arianism over the nature of the Trinity. It was the decision of the council, formalized in the Nicene Creed, that God the Father and God the Son were consubstantial and coeternal and that the Arian belief in a Christ created by and thus inferior to the Father was heretical. Arius himself was excommunicated and banished. The council was also important for its disciplinary decisions concerning the status and jurisdiction of the clergy in the early church and for establishing the date on which Easter is celebrated.

Constantinople I was called primarily to confront Arianism, the heresy that had been subdued only temporarily by the First Council of Nicaea. It reaffirmed the doctrines of the Nicene Creed and to depose Maximus, the Arian patriarch of Constantinople. They also condemned Apollinarianism, a position that denied the full humanity of Christ. The council defined the position of the Holy Spirit within the Trinity; it described the Holy Spirit as proceeding from God the Father, coequal and consubstantial with him. It also confirmed the position of the patriarch of Constantinople as second in dignity only to the bishop of Rome.

Bernard is one of the few Oneness writers who does not directly attribute the doctrine of the Trinity to Satan. He seems aware of the fact that the Oneness position avoids the supposed "philosophical language" by basically ignoring the issue that was faced squarely at Nicea and Chalcedon.
[Quoted from "The Trinity, the Definition of Chalcedon, and Oneness Theology" by James White, http://www.aomin.org/CHALC.html]

It was thus ready at hand, when, in the early years of the fourth century, the Logos-Christology, in opposition to dominant Sabellian tendencies, ran to seed in what is known as Arianism, to which the Son was a creature, though exalted above all other creatures as their Creator and Lord; and the church was thus prepared to assert its settled faith in a Triune God, one in being, but in whose unity there subsisted three consubstantial Persons. Under the leadership of Athanasius this doctrine was proclaimed as the faith of the church at the Council of Nice in 325 A.D.
[Quoted in: "The Biblical Doctrine of the Trinity" by Benjamin B. Warfield
http://members.tripod.com/~Michael_Bremmer/trinity.htm]

It was originally an attempt to deal with a group of people who saw Jesus as a supernatural entity somewhere above a human being but below God. At the council of Nicea, it was decided that this was wrong. In response, Nicea formulated a concept that has come to be called the Trinity. [Quoted in: "More about Christian Beliefs: The Trinity"
http://geneva.rutgers.edu/src/christianity/trinity.html]