CCC 242 Following this apostolic tradition, the Church confessed at the first ecumenical council at Nicaea (325) that the Son is “consubstantial” with the Father, that is, one only God with him.1 The second ecumenical council, held at Constantinople in 381, kept this expression in its formulation of the Nicene Creed and confessed “the only-begotten Son of God, eternally begotten of the Father, light from light, true God from true God, begotten not made, consubstantial with the Father”.2
1 The English phrases “of one being” and “one in being” translate the Greek word homoousios, which was rendered in Latin by consubstantialis.
2 Niceno-Constantinopolitan Creed; cf. DS 150.