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 faith 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, originating in southern France during the fifth century. Each of these creeds, as well as some of the other credal or confessional statements, came into being to deal with what was perceived to be a concrete threat to Christian unity in teaching.
Creeds are important. While they are not divinely originated, they are designed to explain what God has revealed, from the Scriptures, about Himself, His creation, and His redeemed people, the Church. As such, 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:
I believe in one God the Father, Almighty, Maker of heaven and earth, and of all things visible and invisible.
And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Ghost of the Virgin Mary, and was made man, and was crucified also for us under Pontius Pilate; He suffered and was buried; and the third day He rose again according to the Scriptures; and ascended into heaven, and sitteth on the right hand of the Father; and He shall come again with glory to judge the quick and the dead; whose kingdom shall have no end.
And I believe in the Holy Ghost, the Lord and Giver of life, who proceedeth from the Father and the Son; who with the Father and the Son together is worshiped and glorified; who spake by the Prophets. And I believe in one holy catholic and apostolic Church.I acknowledge one Baptism for the remission of sins; and I look for the resurrection of the dead, and the life of the world 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 essense, 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.