We begin our worship with the Greeting which is "The grace the Lord Jesus Christ, the love of God, and communion of the Holy Spirit be with you all." 2 Corinthians 13:13
The affirmation of a triune God has come out historically in the creeds of the church. All three creeds - Apostles', Nicene, and Athanasian - were written as statements of faith. The context was to affirm the church's understanding of what the Bible teaches, as well as combating heresy in the church.
It was most important to affirm the importance of all three persons in the Trinity. In the Athanasian Creed we have these words: "The Father eternal, the Son eternal, and the Holy Ghost eternal. And yet they are not three Eternals, but one Eternal." The church wanted to affirm that no one person of the Trinity was less than another.
In preparation for Sunday's worship, I would invite you to read through all three creeds which can be found here as part of the Book of Concord, the confessional writings of the Lutheran Church.
May God's blessing be upon your contemplation of the great God we have and on your worship this Sunday!