What you believe in is what you will ultimately invite in your life.
The universe is an open field of potentiality. What you can imagine, what you want to believe, you start creating. Your intent, your approach, as well as all that you attract in your life, including people and situations, work towards re-affirming your belief. Once you make a choice, you give up your right to the choice lying at the other end of the spectrum.
Ultimately, what we choose to believe is a decisive part of how we choose to experience life. So be careful of what you choose to believe in; it may well become a self-fulfilling prophecy!