I was conversing with another Christian when I wrote this, not atheists. But, since you have seen fit to butt in, We do indeed know such things. We speak of the things that we know with absolute certainty. Much of what science thinks it knows may be right, or may be shown to be wrong at some later point. Science is never exactly finished, and good scientists realize it, because they've seen many a scientific conjecture become disproved and redefined later on.
With regards to God, however, you do indeed know that God is, though you reason within yourself that if you rationalize away God (which isn't hard to do) then all is and will be well. By this mechanism we can ignore the will of God and dull our consciences. Having done this, we can go on to create a world, with our thoughts and actions, where things are not well -- not well at all. Nazis and communists loved science and hated God, refusing to believe God. The same created a living hell on earth when they came into power.
And the moral of this story is: God is real, and knowing and obeying the will of God is good for humankind and for the whole world. Also, believers are not opposed to the practice of science and its usefulness.