Quickly - not in my words but see the highlighted section below

God’s three faces
The term Trinitas (Latin) was coined by the early church theologian Tertullian (A.D. 160-225) and probably first used in the sense of the coexistence of Father, Son and Holy Spirit in the unit of the Godhead by Theophilus, Bishop of Antioch in Syria (A.D. 169-177). While not a biblical term, The Trinity represents the crystallization of New Testament teaching. In writing his first letter to the Corinthians in about A.D. 55, just two decades after Christ’s death and resurrection, St. Paul correlates Spirit, Lord and God (1 Corinthians 12:4-6). There is a similar correlation in the benediction of 2 Corinthians 13:14 and in the trinitarian baptismal formula of Matthew 28:19.

The church’s confession of faith originated as a baptismal formula. "In the Name of the Father, Son, and Holy Spirit asserts that God reveals himself in a threefold manner because he is a triune God. The doctrine is founded on the events of revelation in which the living God has disclosed himself to the world and manifested his determination to establish communion with humankind" (Encyclopedia of the Lutheran Church).

Creation, redemption, reconciliation
"When the church, on the basis of the prophetic and apostolic witness, confesses one God it confesses its faith that the creator at the beginning of time and the re-creator at the end and the redeemer at the center of time is one God. And again, when the church, in obedience to the same witness, worships this one God by three distinct names, it recognizes and acknowledges the difference between creation, reconciliation, and redemption, and it confesses in the one God the three distinct persons, Father, Son and Holy Spirit" (Encyclopedia of the Lutheran Church).

Who is God?
ELCA Lutherans believe that God reveals who God really is to us. Therefore the Christian church must confess its faith in the essential Trinity. God is one God, revealed in three persons. Article 1 of the Augsburg Confession affirms the doctrinal decisions of the fourth century that deal with the oneness of the divine substance which is God, and the difference of the three persons (sometimes spoken of by their functions as Creator, Redeemer, Sanctifier), declaring them fundamental for the faith of the Reformation. ELCA Lutherans fully subscribe to these confessions presented by the reformers to Emperor Charles V in 1530 in Augsburg, Germany.

Of the Godhead Article 1 of the Augsburg Confession says, "We unanimously hold and teach, in accordance with the Council of Nicea, that there is one divine essence which is called and which is God, eternal, incorporated, indivisible, of infinite power, wisdom, and goodness, the maker and preserver of all things, visible and invisible. Yet there are three persons, of the same essence and power, who are also co-eternal: the Father, the Son and the Holy Spirit."

The Evangelical Lutheran Church in America, together with the other 135 Lutheran World Federation* member churches, therefore, are Trinitarian churches, understanding that God has chosen to reveal God's self in triune fashion so that we might better know, understand and witness to God’s activity in the world. With Western Christian churches, we celebrate the Sunday after Pentecost as Trinity Sunday