Becoming cognizant of the fact that our limited ability to know the world will always fall short of the totality of what exists, faith in God may be defined as a fundamental trust that our lives and our reality are meaningful and purposive. In the face of uncertainty, there is no possibility of such an assertion without faith.
In a sense, to believe in meaning is to believe in God. Similarly, not to believe in God is also to not believe that the meanings and purposes that one's life possess are fundamentally grounded in reality. While believer and unbeliever can understand their lives and reality as a whole to be meaningful, it is only through God that such a belief can be justified.
Because our ability to know is limited, God can neither be irrefutably proved or disproved to the satifaction of all. Therefore it remains a matter of belief rather than fact on whether we posit reality as meaningful or not. How we ultimately regard reality invites us to make a decision. To decide not to choose in itself means that one is only choosing not to trust.
Because non-believers decision is to not trust in the ultimate meaningfulness of life, it would be a misnomer to say that their position is based on faith. Yet because of the uncertainty inherent in our limited ability to know, it cannot be stated that believing to trust is any more irrational than choosing to not believe.
A believer's decision to trust in God is based in the totality of his or her experience of reality. It is not just based on limited concept of reasoning that has been so successful in modern scientific endeavor. It is much more based in the depth of experience and the qualitatively different ways we have of experiencing such as through or appreciation, our emotions, our actions, our sense of aesthetics, and our will. In the classical sense, not just cognition, but all of these different aspects through which we come to know ourselves and the world were regarded as rational.
As it is only through deciding to open ourselves open to another that we experience love, so it is only through the decision to trust in God that we become aware of God. That believers are in fact aware of God's presence in their lives demonstrates the rationality of the choice.