All theories are just assumptions of cause and effect relationships that seem plausible and provide reasonably useful prediction for how things might work so we can plan accordingly. It is prudent to be always aware that even the most trustworthy theory might prove inapplicable in a particular situation which demands an innovative approach.
We act we act on our beliefs . We act on what we believe is our best interest and we do what we believe is the best way to achieve it. A prudent person will make every effort to gather all available relevant information and evaluate its reliability as objectively as possible. But ultimately people will be driven by their passions and desires and will act on what they believe is genuine knowledge about how things work. The strength of our beliefs and factored by the relevance of our knowledge will make our actions a part of reality. What is more important it will have more significant effect on that part of reality the is closest to us and affects us the most. Therefore faith is a substantial part of reality and the most important force and tool to achieve our goals. We live because we believe. Or more correctly - the way we live is a reflection of our beliefs.