What are the longterm implications for mankind if God belief eventually fades away?

God will never 'fade away' He is the I AM, which is a continuing state of being. But if there are not enough believing in him he will do what he did with the wicked world of Sodom and Gomorrah, The people in Noah's time and what he promised to do soon. Destroy all he so lovingly and carefully created and start over with a new heaven and earth.