Augustine spoke of the “book of nature” which powerfully testifies to all of the Creator. In the book Sermons: 51-94 (New City Press, 1991) on p. 225-226, Augustine is recorded as saying,

Some read a book to find God. But there is a great book: the spectacle of what has been created. Look upwards and downward; pay attention and read. In order to enable you to read that book, God did not write in letters with ink but he placed what is created itself in front of you. Why do you seek a louder voice? Heaven and earth are crying to you: God made me.

Augustine explains that the human mind, created in the inmage of God, is enabled to perceive the testimony of creation. This “book of nature” is the general revelation that Paul describes in Rom 1:19-20. It is universally available to everyone, everywhere for all time. We are ultimately not responsible for what we don’t know; only for that which we do know. And God has graciously given knowledge of his existence to everyone. Everyone is accountable for the knowledge that God exists – a knowledge that some choose to willfully suppress (Rom 1:18). The “book of nature” is accessible to all, including the illiterate.