From Alistair E. McGrath’s book, Mere Apologetics:
Now there is much more to the Christian idea of faith than believing certain things are true. For Christians, faith is not merely cognitive (“I believe this is true”), but also relational and existential (I trust this person”). It is not just believing that God exists, but discovering that this God is wise, loving, and good—and choosing to commit ourselves to this God as a result. As C.S. Lewis once remarked, you are not faced “with an argument which demands your assent, but with a Person who demands your confidence.”
Faith is thus about trust in someone, not just a belief that he or she exists. This point was made by Danish writer Soren Kierkegaard (1813-55), who emphasized that true faith in God was a “qualitative leap” from one way of existing to another. Christian faith is not the mere addition of one extra item to our inventory of the contents of the world—that is, God. It is about realizing and embracing the new “mode of existence” this trust makes possible.