How do we know the law cannot justify?

The law was given to demonstrate humanity's sinfulness and inability to achieve righteousness on their own. The Apostle Paul clearly states that by the deeds of the law, no flesh shall be justified in God's sight. This is underscored in Romans 3:20, which affirms that the law provides the knowledge of sin, not a means to righteousness. The law's purpose was to lead us to Christ, as it shows our need for a Savior. Any attempt to earn God's favor through adherence to the law undermines the grace offered in the gospel of Jesus Christ.