Being all-powerful and all-knowing, God could have chosen to communicate with His people any way He wanted to. But He chose a few particular means.
According to Psalm 19, God speaks through His creation: The heavens declare the glory of God, and the sky above proclaims his handiwork. Day to day pours out speech, and night to night reveals knowledge (v. 1-2). He also speaks through the written word: The law of the Lord is perfect, reviving the soul; the testimony of the Lord is sure, making wise the simple (v. 7).