What you believe in is what you will ultimately invite into your life.
The universe is an open field of potentiality. What you can imagine and what you want to believe, you will 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 which lies 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 might just become a self fulfilling prophecy!