That's how the Creed begins. Our life in Christ begins in faith. This is more than merely acknowledging certain theological truths. It is to surrender one's life to these truths. "Faith without works is dead," (Jms. 2:26). Therefore true faith cannot be merely intellectual. It has to find its way into the heart. It must lead one to a radical break with the old life of sin and a new life in Christ. It must lead to love; love of God, first and foremost, and love of neighbor.
Faith typically comes in stages. It is more often a progression, not an event. This typically begins with inquiry. One begins to sense his need for God. This usually begins with an understanding that something is not right and there needs to be a change. We typically try to make that change in a variety of ways: a change in diet, exercise, routine, or in more extreme cases, a job, or significant relationship. But the true problem lies deep within us. When we finally realize that, we are ready for more.
Our period of inquiry then leads to the seeking of God. At first, this may come in the form of seeking the answers to the deep questions of life: "Who am I?" "How did it all begin?" Why am I here?" What is the meaning of life?" Why is there suffering?" "Where will it end?" "What happens when we die?" As we explore these questions, we are led inevitably to God. This is where the Creed comes in. It tells us who God is and how we are able to get to know Him.
The next step in our progression is repentance. As we come to understand who God is, we see that we fall terribly short of His standard. This shortcoming is called sin. It is innately in us from birth and manifests itself in various ways. Some people do things that are considered very bad such as drink too much, live sexually immoral lives, steal, lie, or even kill. Others live apparently good lives but are consumed with their own pride and self-seeking. Either way, until they come to know God, they are ruled by sin.
Repentance is to turn away from sin and turn towards God. It is to change our mind and to change our lives. It is to think entirely different. Whereas we used to indulge our sins, thinking this would give us true happiness, we now realize that sin is destroying us and any chance we have at true happiness. We see God and His ways as the only way to freedom- and joy. Therefore, we learn to hate the things we once loved (sin), and we now love the One we had been fleeing before (God).
At this point, we have already begun to exercise faith. It's beginnings are subtle, but we can identify it in our actions. When we begin this work of repentance, faith has already gripped us. Otherwise we would not believe there to be a need for change. As we continue in faith, we learn to know God. We come to love Him, and we pursue a life of obedience to Him as a result.
This is what it means to say, "I believe." It means we have personally appropriated the things that come afterward in the Creed. It means that we have come to know God and we continue to allow God to do the ongoing work of transformation necessary to make us holy just like Him.