Faith that Jesus is the Christ and that He was risen from the dead is the foundation of Christianity. Romans 10:9-10 states "that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation."
A person desiring to become a Christian must confess the faith that is in his heart. An Ethiopian eunuch in the Scriptures was asked if he believed with all his heart, he responded with a beautiful, simple confession of his faith. "I believe that Jesus Christ is the Son of God." Acts 8:37
Faith needs to be expressed.
"Whoever confesses that Jesus is the Son of God, God abides in him and he in God." 1John 4:15