The Second Ecumenical Council was convened in the year 381 and consolidated the victory of Orthodoxy attained in the year 325 at the First Ecumenical Council.
During the difficult years which passed after the acceptance of the Nicene Symbol of Faith (Creed), the Arian heresy developed new offshoots. Under the guise of struggle against the Sabellian heresy, which taught about a blending of the Hypostatic Persons of the Father and the Son [as mere aspects or modalities within the Trinity], Macedonius began to employ the word “homoiousios” “of similar essence” [in contrast to the Orthodox teaching of “homoousios”, “of the same essence”] regarding the essence of the Son and that of the Father.
This formula still presented a danger because Macedonius presented himself as a struggler against the Arians, who used the term “like the Father.” Besides this, the Macedonians, being semi-Arians, depending on conditions and advantages of the moment, sometimes inclined towards Orthodoxy, sometimes towards Arianism. They blasphemed the Holy Spirit by suggesting that He was not “of the same essence” with the Father and the Son.
A second heretic, Aetius, introduced the concept “anomoion” (“different in essence.”) He said that the Father has a completely different essence from that of the Son. His disciple Eunomios taught a hierarchical subordination of the Son to the Father, and of the Holy Spirit to the Son. Everyone who came to him was rebaptized into the “death of Christ,” denying Baptism in the Name of the Father and of the Son and of the Holy Spirit, which is commanded us by the Savior Himself (Mt. 28:19).
A third heresy arose from the teachings of Valentius and Ursacius at the Arimonian Council. They attempted to deceive the Orthodox bishops, proclaiming that the Son of God is from God, and is in the likeness of God the Father, and is not a created being as the Arians taught. The heretics did not wish to use the term “one in essence” in describing the relation of the Son to the Father, saying that the word “essence” is not found within the Holy Scripture. Besides these three main heresies, there were also many other false teachings. The heretic Apollinarios said, “The flesh of the Savior did not have a human soul or reason. The Word of God took the place of the absent soul; and Divinity remained dead for three days.”
In order to refute these heretical opinions, the holy Emperor Theodosius the Great (379-395) convened an Ecumenical Council at Constantinople, at which 150 bishops were present. Upon investigation by the Holy Fathers it was proposed that a Creed which holy Pope Damasus had sent to Bishop Paulinus of Antioch should be read. This appears to be the so-called Sirmean Creed, drawn up by Potamius of Lisbon, who participated in a pro-Arian Council at Sirmium in 357, but changed his opinions two years later. The document was a vain attempt to please everyone by not mentioning the terms ousia (essence, or substance), homoousios (identical in essence, or substance), and homoiousion (similar in essence, or substance), "by which the minds of many are perturbed." It said that there "ought to be no mention of any of them at all, nor any exposition of them in the Church." By not defining the Church's teaching clearly, the document is ambiguous, neither endorsing the various heresies it contained, nor the teaching of the Church.
An Ecumenical Synod is convened primarily to deal with false teachings and refute them by proclaiming the faith handed down to us by the Apostles and the Holy Fathers. To use the definition of St. Vincent of Lérins: "that which has been believed everywhere, always, by everyone." There can be no compromise between truth and falsehood.
After the document was aloud, the Holy Fathers rejected the false teaching of Macedonius, and unanimously affirmed the Apostolic teaching that the Holy Spirit is not a creature, but is rather the Life-Creating Lord, Who proceeds from the Father, and is worshipped and glorified with the Father and the Son. In order to combat other heresies, of the Eunomians, Arians and Semi-Arians, the Holy Fathers reaffirmed the Nicene Symbol of Faith.
In the Symbol (Creed), accepted by the First Ecumenical Council, the divine nature of the Holy Spirit was not addressed, since at that earlier time [in 325] heresies against the Holy Spirit had not become widespread. Therefore, the holy Fathers of the Second Ecumenical Council added to the Nicean Symbol its eighth, ninth, tenth, eleventh and twelfth sections. They definitively formulated and affirmed the Nicene-Constantinople Symbol of Faith, which is used even now by all the Orthodox Churches.
The Second Ecumenical Council also established the norms for ecclesiastical courts [Canon VI], and it decided to receive those repentant heretics who were properly baptized in the name of the Holy Trinity through Chrismation, but those baptized with a single immersion were to be received as pagans.