John Calvin beautifully articulates what it means that “the heavens declare the glory of God”:
[God] has also displayed his perfection in the whole structure of the universe. So he is constantly in our view and we cannot open our eyes without being made to see him. His nature is incomprehensible, far beyond all human thought, but his glory is etched on his creation so brightly, clearly and gloriously, that no one however obtuse and illiterate can plead ignorance as an excuse. So with absolute truth the Psalmist exclaims, ‘He wraps himself in light as with a garment’ (Ps. 104:2). It is as though he was saying that when God created the world for the first time, he put on outer clothes. He hung up gorgeous banners on which we see his perfection clearly portrayed…
Wherever you look, there is no part of the world however small that does not show at least some glimmer of beauty; it is impossible to gaze at the vast expanses of the universe without being overwhelmed by such tremendous beauty. So the author of Hebrews sensitively describes the visible world as an image of the invisible (Heb. 11:3). The superb structure of the world acts as a sort of mirror in which we may see God, who would otherwise be invisible.
John Calvin, The Institutes of the Christian Religion ed. Tony Lane and Hilary Osborne, p. 32-33