God exists in every mind because of fear. Fear from being stolen, killed or raped. Our mindsets have become like this from childhood from the preaching we have been provided at home and school based on our religion.
Aside from fear, God exists in people's minds because they are very grateful to Him for all the blessings He had showered upon us. God is also in our mind because we seek for guidance and protection. God is always there for us and in return, God should also be in our minds.