Psychologically we can define beliefs as the subjective truths or biases that we have ingested and formulated for ourselves to efficiently function in the real world. Beliefs may help us to be efficient but may not be effective always because they are polluted. Beliefs are highly influenced by our biases that we created for ourselves all throughout life. Beliefs must evolve and meant to be changed as we become mature in life.
In a way a belief is nothing but a thinking process that you have created to save your mental resources. That means when you come across some situation in life your response would be mentally less stressful when it is aligned with your beliefs and more effort full if it is not a line with your beliefs.
*Do read the Disclaimer