C.S. Lewis says of this psalm,
People often say, "How can I believe in God I cannot see?" Good question. Answer Psalm 19. Basically David says the heavens declare the glory of God, and the skies proclaim the work of his hands.
David says the same thing in Psalm 8, "When I consider Your heavens, the work of Your fingers, the moon and the stars, which You have ordained. What is man that You take thought of him, And the son of man that You care for him."
Paul says something similar in Romans 1 when he states, "For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse."
He is basically saying there are two options to explain the world around us and where we as humans came from. 1.) God created the world and human beings as the crown of His creation. 2.) It happened by chance or what some call evolution.
The problem with evolutionary theory is that they can never come up with what started the evolutionary process. Something cannot come out of nothing. For instance some call it the "Big Bang". The problem with this theory is that to get a "bang", you need a "banger".
So what can we conclude? In His creation God fully reveals who He is, His power, creativity and beauty. Finally, God's revelation did not end with Nature. God sends His Son Jesus in the flesh, once and for all to show specifically who He is. In Jesus he reveals his love for mankind, when Jesus dies on a cross for us. Jesus reveals the purpose for why we are here. To give glory to God and worship Him alone, the Creator of the sun, the moon and the stars.
So God reveals Himself in two very clear ways. Through Creation, and through His Son. We can either believe in God's revelation of Himself, or choose to believe in something else. For the life of me, I cannot see why anyone would make that choice.