The second word of The Apostles' Creed is critical: believe. What does it mean to believe? What is faith?
The author of Hebrews said, "Now faith is being sure of what we hope for and certain of what we do not see. This is what the ancients were commended for. By faith we understand that the universe was formed at God's command, so that what is seen was not made out of what was visible." (Hebrews 11:1-3) Then the writer to the Hebrews goes on to give several examples of people of faith from the Hebrew Scriptures. He shows us how faith involves trust and results in obedience.
Real faith involves understanding and assenting to certain truths. "And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him." (Hebrews 11:6) Faith begins with understanding certain things about God and assenting to that information as true.
Many well-meaning Christians try to share their faith with others, but very often they expect others to come to the same conclusions they have with very little information upon which to base an intelligent decision. I remember preaching in a juvenile detention facility in California a number of years ago. I gave a simple presentation of the Gospel and shared with the young women present how they could receive forgiveness of sin through Christ. After a time of prayer, several of these young women raised their hands indicating that they had prayed to accept Christ into their lives. Afterward, one of the Christian counselors accompanying me was talking with one of these young women in the facility. She learned that this young woman wanted forgiveness for some bad things she had done but she did not believe that Jesus was the unique Son of God. The young woman was from a Muslim background. Obviously, there had been inadequate preaching on my part that led to some confusion in the mind of this young woman. Perhaps she was taking some important spiritual steps, but clarification was needed. Christian faith begins with a clear understanding of who God is: Father, Son and Holy Spirit. We must give intellectual assent to these central truths if we are going to call ourselves Christians.
However, is acceptance of such truths on an intellectual level enough? James says, "No." He writes, "You believe that there is one God. Good! Even the demons believe that--and shudder." (James 2:19) I think James was telling us that the demons have the best theology. They know who God is and they assent to that truth, even to the point of having a kind of fear of God. However, the one thing the demons do not do is bow their knees and surrender control of their lives to God.
Our faith must be more than intellectual assent if it is to be true Christian faith. We must go beyond understanding and assenting to trusting and committing. For example, I can assent to the propositional truth that planes fly from Washington, D.C. to San Diego, California. I have family in San Diego and often think about going there. However, I have not demonstrated complete faith until I purchase an airline ticket and get on the plane to actually go to San Diego. That action demonstrates real faith, real trust in the power of an airplane to fly. Just so, real faith in God involves all three of these elements: understanding, assent, and most importantly: trust.
I like what C. S. Lewis has to say about faith in Mere Christianity....
Now Faith, in the sense in which I am here using the word, is the art of holding on to things your reason has once accepted, in spite of your changing moods. For moods will change, whatever view your reason takes. I know that by experience. Now that I am a Christian I do have moods in which the whole thing looks very improbable: but when I was an atheist I had moods in which Christianity looked terribly probable. This rebellion of your moods against your real self is going to come anyway. That is why Faith is such a necessary virtue: unless you teach your moods "where they get off," you can never be either a sound Christian or even a sound atheist, but just a creature dithering to and fro, with its beliefs really dependent on the weather and the state of its digestion. Consequently one must train the habit of Faith.