In Scripture, to know God usually means to have a close and saving relationship with him (see 2 Corinthians 5:16; Galatians 4:9; Philippians 3:8, 10). Here, however, they knew God means that people knew about God. All people have some understanding of God through creation, but they do not do what is right based on that knowledge. Instead of learning more about God, they worship gods they create themselves.