What did George Washington mean when he said this?

"It is impossible to govern the world without God and the Bible. Of all the dispositions and habits that lead to
political prosperity, our religion and morality are the indispensable supporters. Let us with caution indulge the
supposition that morality can be maintained without religion. Reason and experience both forbid us to expect
that our national morality can prevail in exclusion of religious principle.