In what do you believe? I mean, if someone asked you about your spirituality, or your faith, or your reason for going to church, what do you say? Most of us do the things that we do for a reason, and I suspect that, for the average person, that reason is rather utilitarian. In other words, we do it because of the perceived benefit to us. Ultimately, as a song from the movie, "Car Wash" went, "You've got to believe in something...."
The early Christian Church developed statements which defined what we believe. They are called "Creeds" or "Symbols," and they lay out the essentials of what it means to be an orthodox ("right-believing") Christian, as opposed to being a heterodox (heretical) one. Pretty much all of the historic branches of Christianity accept the first three universal or ecumenical creeds, the Apostles' Creed, the Nicene Creed, and the Athanasian Creed. Every Christian church either directly confesses these creeds, or relies upon the concepts defined by those creeds for their own "statement of faith."
The Apostles' Creed, which reached its current form by the eighth century, was developed from what was known as the "Old Roman Creed," which came into being during the third century. The Nicene Creed, developed as the Church wrestled with questions regarding the nature of Christ during the fourth century, was the defense against Arian attempts to portray Christ as a superior creature, but not God. The Athanasian Creed a Latin expression of faith, originated in southern France during the fifth century. Each of these creeds, along with other creedal or confessional statements, came into being to deal with what was perceived to be a concrete threat to Christian unity in teaching.
Creeds have been used as teaching tools, as baptismal confessions, as well as statements for the defense of the faith. It's probably easier, for example, to memorize the Nicene Creed than it is to memorize all of the passages of Scripture that address the issues contained within it, for most of us, and it's easier to put in your pocket. Here, as an example of what I mean, is a copy of the Nicene Creed:
“We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things, seen and unseen.
And in one Lord, Jesus Christ, the only Son of God, begotten from the Father before all the ages, God of God, Light of Light, true God of true God, begotten, not made, of one being with the Father; through whom all things were made. For us human beings and for our salvation, he came down from the heavens, and was incarnate of the Holy Spirit and the Virgin Mary, and became a human being. He was crucified for us under Pontius Pilate; He suffered death, and was buried. On the third day He rose again in accordance with the Scriptures; he ascended into the heavens and is seated at the right hand of the Father. He is coming again in glory to judge the living and the dead. There shall be no end to His kingdom.
And in the Holy Spirit, the Lord and Life-Giver, who proceeds from the Father and the Son, who with the Father and the Son is worshiped and glorified, who spoke through the Prophets.
In one holy, catholic, and apostolic Church.
We acknowledge one Baptism for the forgiveness of sins; we look for the resurrection of the dead, and the life of the age to come. Amen.”
Maybe your church confesses a creed; maybe it holds to the position, "No creed but Christ; no belief but the Bible." Either way, you have an idea of what it means to be a Christian, and what it does not mean. That, in essence, is what a creed does for us. We can try to "re-invent the wheel," creating a statement of what we believe based on our own insights and issues, or we can join with the generations of Christians stretching back to the third and fourth centuries, reflecting a faith that goes back even further. Otherwise, people can define your faith for you, as many try to do, usually in ways we don't like. For the fledgling Lutheran Church, the inclusion of these creeds within the Augsburg Confession and its Apology, or, Defense, conveyed that they were not making a radical departure from what it meant to be a Christian and part of the Universal Church. Martin Luther argued that, contrary to being heretical, his teachings were in perfect alignment with the earliest teachings of the Church, and that it was Rome that had strayed, thus requiring faithful ministers to call her to account for her good, and for the benefit of believers.
In a larger sense, in fact, that is the message of the entirety of the Augsburg Confessions. Philipp Melanchthon hoped that Rome would understand, and accept, what the Lutheran pastors and civic leaders were saying, and would embrace these positions. Sadly, that did not happen, and the division between the two groups hardened, to the point that the Lutheran believers were forced to choose between obedience to Rome, and obedience to the Gospel.