1. The Seven Great Ecumenical Councils of Early Christianity (I)
Nicaea (325 CE) Declared Jesus to be homoousios (of the same being as the Father) not homoiosios (of a similar but not the same being as the Father (Arianism). 50 copies of the Christian scripture were also commissioned at this council. The Council suggested that Easter should always be celebrated on a Sunday (1st Sunday after the first full moon after the vernal equinox) and that it should always come after the Jewish Passover. It also declared that baptized Christians who apostatized but reconverted need not be rebaptized
Constantinople I (381 CE) Confirmed the Jesus was fully divine and stated that the Holy Spirit was likewise fully divine. Declared that Apollinarianism (Jesus not fully human) was in error as was Sabellianism (The trinity comprised three persons not three modes of one person)

2. The Seven Great Ecumenical Councils of Early Christianity (II)
Ephesus (431 CE) Declared that, in Jesus, two natures were perfectly joined in one person and that the person that Mary brought into the world had a fully human nature and a fully divine nature joined in the one person. Mary was therefore Theotokos, God-Bearer, and could be called Holy Mary, Mother of God. Calling Mary the Mother of God was making a statement about Jesus, not a statement about Mary This declaration countered Nestorianism which held that two persons were joined together in Jesus. It confirmed the decision of the regional Council of Carthage regarding Pelagianism
Chalcedon (451 CE) Confirmed and further explained Ephesus and settled some organizational issues
Constantinople II (553 CE) Further explained and confirmed Ephesus and declared some of the teachings of Origen regarding Jesus to be in error

3. The Seven Great Ecumenical Councils of Early Christianity (III)
Constantinople III (680/81 CE) Declared that, since Jesus had a fully human nature and a fully divine nature perfectly united in one person, Jesus had two wills (human and divine) and not the one will taught by the Monothelites
Nicaea II (787) Declared that, since Jesus was a human being who walked the earth, and that he told us to call God father and that Scripture indicated that the Holy Spirit appeared both as a dove and in tongues of fire, the actions of the Iconoclasts that forbade any portrayal of the members of the Trinity were contrary to church teaching
Since the split between the Roman Church and the Eastern churches in 1054, the Roman Church has held a number of Councils, most recently, Vatican II but the Eastern churches have held none until last month. Ten churches attended four (most notably Russia) did not