"Father, protect hem ( the disciples) by the power of your name so that they may be one, as we are one" (John 17,11).The 1st Ecumenical Council was held in Nicaea in 325 in the reign of Constantine the Great.

Some of the 2nd  and 3rd generation Christians began to have doubts about what they had been taught about Jesus. Some Christians with a Greek background had a hard time beliving that Jesus was human as well as divine, because in Platonic thought the spirit was all-important; the body was only a prison from which one desire to escape. Heresies developed from a unity of this kind of Platonidc thought and Christianity.  Apostle John, an old man and perhaps the only surviving apostle, wrote between A.D 85 and 90 his letter 1 John to tell his "dear children" that he walked and talked with Jesus, saw Him heal, heard Him teach, watched Him die, met Him arisen, and saw Him ascend. A particular  false teaching was called Docetism (from a greek word meaning "to seem") held that Jesus was actually a spirit who only appeared to have a body. Another heretical teaching, Gnosticism ( from a Greek word meaning "knowledge") held that  all physical matter was evil, the spirit was good and found hard to believe in a Savior who was fully human. As an eyewitness of Christ, John wrote authoritatively to give new generation of believers assurance and confidence in God and in their faith against false teacher and heresies.

The 1st Ecumenical Council was summoned to remove the confusion which Arius, an Alexandrian priest,  preached that Christ  was created by God  in time, and He was not equal  with God the Father. 380 fathers took part in this Council; it condemned the Arian teaching, confirmed the Symbol of Faith (the Creed), and the time of Easter celebration. At the  Council stood out: St Nicolas of Myra in Lycia,  St Spiridon, St Athanasius, Macarius of Jerusalem,  Eusebius of Caesarea and many from the East; from the West:  Hosius of Cordoba, Theophilus the Goth, Cecilianus of Carthage, and others.

The Council of Nicaea declared the nature of Trinity. In Greek terms it was declared to consist in three hypostaseis in one ousia ( three entities in one being). In Latin, God was said to be tres personas in una substantia ( three persons in one substance). These became classical ways of defining the Trinity, and constituted the achievement of St. Athanasius (296 - 373).

But Arianism, though banished from Orthodoxy, continued and made great inrods in the conversion of Northern Europe in subsequent centuries. ( Further comments in future article "Holy Trinity").