God has proven to man of His general existence through His creation. The Bible says that man is foolish not to believe that there is a God. This is due to man’s sin/rebellion. God will judge foolish man for his rebellion.
18 The wrath of God is being revealed from heaven against all the godlessness and wickedness of people, who suppress the truth by their wickedness, 19 since what may be known about God is plain to them, because God has made it plain to them. 20 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.