The exploration of caves is called spelunking. Caves are dark tight spaces, and you would be foolish to explore one without a really good headlamp. As black and scary as it can get deep in a cave, turning on that headlamp vanquishes the darkness and floods the space with light.
That is exactly what God did in the world and our lives. His presence floods us with light and runs out all the darkness. Light and dark cannot coexist. In the passage, John equates light with truth and darkness with sin. John is telling us that we cannot claim we have no sin once we have accepted the real truth. We cannot continue to walk in darkness once we have encountered the truth of the gospel. Truth changes us in ways that we cannot go back from.
Maybe it is time to truly turn the light on in the dark cave of your life. What would it look like for you to walk into the light of God’s truth…fully accepting him as the true Savior who forgives your sins and purifies you?