Nature is saying something.
Psalm 19:1 explains: "The heavens declare the glory of God." The most basic point of this verse is that the world around us is making God known. "Wherever you cast your eyes," Calvin writes, "there is no spot in the universe wherein you cannot discern at least some sparks of [God's] glory" (Institutes, I.V.1). The theological name for this is "general revelation."
We can learn things about God from what we observe around us, from human interaction, from culture and society. But it won't be enough. Nature's speech is too generic and our reason is too dark. We need a word. We need an interpreter. As Calvin puts it, "it remains for God himself to give witness of himself from heaven” (I.V.13).
If we want to know God, if we want to have communion with him, gazing out at the setting sun, no matter how gorgeous it may be, is not going to do it. Neither will our most meaningful experiences. Nor our deepest friendships. We must have the word of God.
John Piper explains,
To be sure, God is active everywhere in the world today, and we experience his precious power wherever we trust him and do his will. But we will go astray if we make this daily experience of God the basis of our communion with him. We know God for who he is, and meet him as he is, when we meet him through his Word — the Bible. . . .
Therefore, when we seek to enjoy communion with the Lord — and not to be led astray by the ambiguities of religious experience — we read the Bible. From Genesis to Revelation, God's words and God's deeds reveal God himself for our knowledge and our enjoyment. (Understanding Scripture, 47–48)