What does it mean to harden your heart against God's Word?

Hardened hearts are characterized by disbelief and a refusal to obey God's call. In Hebrews 3, the Israelites' failure to enter the promised rest exemplifies this hardness, as they saw God's works but did not believe His promises. A hardened heart becomes resistant to the influence of the Holy Spirit and leads to spiritual decay. The sermon highlights the need for constant vigilance, exhortation among believers, and reliance on God's Word to prevent this spiritual condition, urging Christians to always be receptive to the Holy Spirit's prompting.