How do we know that human effort is ultimately vain?

The declaration in Psalm 39:6 that 'every man walketh in a vain show' illustrates humanity's tendency to overestimate its worth and achievements. In comparison to God’s holiness and righteousness, our best efforts are like 'vanity'—lacking substance and true fulfillment. This teaches us that true value and significance come from our relationship with Christ, not from what we can do on our own. Understanding our inherent emptiness apart from Him drives believers to seek their sustenance in Christ alone.