They think they do. Or more importantly, they hope they do.
You see, if there is no supreme being of the universe, there must necessarily be a superior one. One who is smarter, stronger, more powerful than those he has contact with. There must be one. He will be called "Master".
That is, unless he is restrained by another force, say the state. And the state makes for a very tyrannical master, indeed.
In the United States, God is a legal concept. Rights are given by God, hence the state cannot deny them. All men are equal under God; otherwise, someone will become a slave.
"...to which the Laws of Nature and of Nature's God entitle them... that all men are created... Assembled, appealing to the Supreme Judge of the world... with a firm reliance on the protection of Divine Providence..."