The heavens declare the glory of GOD: and the firmament shows his handiwork.
David acknowledges that there is a GOD, the GOD.
Sometimes when I'm outside, I stare up at the sky and I am amazed. You know when the sun sets and its so beautiful, b/c the entire sky is a mixture of red, orange, and pink? That's when I see the glory of GOD.
Or you can look at the word "heavens" in a broader sense: the universe. At night, w/the human eye we can see the moon, stars, and a couple planets. I can't imagine what David saw when he looked into the night sky. With no city lights to take away the blackness of night. If a descriptive poet like David uses the "heavens and firmament" to compare to GOD's glory how much more magnificent HE must truly be.