There were many serious theological controversies in the early centuries of the Church. At times the Church had to gather in council to decide formally what was genuine Christian faith. One such case was the Arian heresy. The Arians said that Jesus was not truly God, but the first and noblest creature that the Father created.
The bishops of the Church met at Nicaea in 325AD to settle this dispute. In order to be precise they borrowed a term from Greek philosophy (homoousios – consubstantial). The Church maintained its belief that Jesus is fully God, explaining that both he and the Father (and the Holy Spirit) share in the one divine nature or substance. The Son is not a separate or lesser being to the Father. They are three Persons in one God: the Most Holy Trinity.
This key idea is now found in the Nicene Creed which has been prayed by the Church since that time. Although ’consubstantial’ is a technical word, it precisely describes our Christian faith: that Jesus is truly God. Saying it together at Mass affirms our unity in this truth of faith.
Read more: Catechism 242, 262, 467, 663.