How do we know the importance of preaching the gospel?

The act of preaching the gospel is fundamentally about revealing sin and grace. In John 15:22, Jesus indicated that His coming brought knowledge of sin to those who might otherwise cloak their transgressions under the guise of righteousness. Without the gospel, people cannot see their true state before God; they may think of themselves as morally upright. Thus, it is through the gospel that God opens the eyes of sinners, revealing their need for salvation and leading them to repentance. The cross ultimately signals the depth of our sin and the vastness of God's grace, making it imperative that the message of Christ be preached.