Why is it a fearful thing to fall into the hands of the living God?

Falling into the hands of the living God is fearful because it symbolizes being subject to His judgment without the covering of Christ's righteousness. The text in Hebrews 10 emphasizes that those who reject God and do not come to Him through Christ will face eternal consequences. To be judged without an advocate, as we stand naked in our sins, is a terrifying prospect. This emphasizes the necessity of Christ's atoning work, as He took our place and bore our judgment, allowing us to stand justified before God. Without Him, there is no escape from divine justice.