If you’d recited the Creed before the Second Vatican Council in the mid-1960s, the line you indicate would have read: “Of one substance with the Father.” After Vatican II and before 2011, that phrase was: “One in being with the Father.” The Greek word translated all three ways is homoousion, “single essence.” The Latin word is consubstantialis, bringing us to the current translation, consubstantial—a word you probably won’t hear in any context other than reciting the Creed.
Jesus puts it more elegantly when he declares in John’s gospel: “The Father and I are one.” He makes a similar proposal to Philip, when the disciple innocently asks to see the Father: “Whoever has seen me has seen the Father.”
The first Christian leader to use the term consubstantial was Origin (185-254). He insisted: “There is no dissimilarity whatever between the Son and the Father.” He further declared “the power of the Trinity is one and the same” by quoting Saint Paul: "There are diversities of gifts, but the same Spirit; there are diversities of administrations, but the same Lord; and there are diversities of operations, but it is the same God who worketh all in all.”
The Council of Nicaea (325), dominated by Athanasius, eliminated any hint of subordination within the Trinity. Meanwhile Arius and his followers, who questioned the equal natures of Father, Son, and Spirit, were branded heretical.
Expressions of the single essence of God became the matter of many early homilies. Irenaeus (130-202) trusted that “When Christ comes, God will be seen by men.” Peter Chrysologus (400-450) affirmed that God becomes known to us in being born for us. Bernard of Clairvaux (1090-1153) preached: “In the fullness of time, the fullness of divinity appeared” in Bethlehem.
Some church fathers took consubstantiality a radical step further. Hilary (315-368) proposed: “We are all one, because the Father is in Christ, and Christ is in us…. With Christ we form a unity which is in God.” Cyril of Jerusalem (315-386) went so far as to say: “If we have given up our worldly way of life… it must surely be obvious to everyone that … our nature is transformed, so that we are no longer merely men, but also sons of God, spiritual men, by reason of the share we have received in the divine nature.” Augustine (354-430) dared to speak the phrase that still stuns us: “God became man so that man might become God.” Complete communion with God remains the goal.
Scriptures: John 1:1-5, 14; 5:19-30; 14:7-11; 17:20-26; Romans 13:14; 1 Corinthians 12:4-6; Galatians 2:19-20
Books: The Trinity: Insights from the Mystics, by Anne Hunt (2010)
The Trinity: An Introduction on Catholic Doctrine of the Triune God, by Gilles Emery, O.P. (2011)