What does 1 John 2:9 mean?

Here, John reveals the new commandment hinted at in the previous verses. His statement is direct, simple, and easy to understand: those who claim to follow Christ, but demonstrate hatred for fellow Christians, are actually walking in darkness. It is impossible for one who hates other believers to honestly claim to have fellowship with Christ. This is stated in strong, clear terms.

In Hebrew thinking, "light" was the ultimate ideal. All good things were described in terms of "light," all evil things in terms of "darkness." So, it's not surprising to see the concept of "being in the light" used often in John's letter. God is light (1 John 1:5), we are to walk in the light as Christ is in the light (1 John 1:7), the true light is already shining (1 John 2:8), and whoever loves his brother abides in the light (1 John 2:10).

The Gospel of John refers to Jesus as "the Light" more than twenty times, seven of those in chapter 1 alone. In the book of Revelation, John also notes that God Himself will be our source of light (Revelation 22:5). Rejecting that light comes with consequences, both in this life and in eternity.