What does Isaiah 5:21 mean?

Isaiah next declares "woe" to those who are arrogant about their own understanding. This verse fits the pattern of the proverbs of Solomon. The people in Jerusalem and Judah had fallen into a common human trap: wanting to be the source of all the goodness in their own lives, including all knowledge. Sin often convinces us that if we don't like something, or cannot fully understand it, it must be wrong. That makes us the ultimate judge of good and evil (Isaiah 5:20) and ignores our own weaknesses (Isaiah 55:8–9).

Solomon's proverbs repeatedly emphasize that true wisdom and knowledge begins with the fear of the Lord (Proverbs 1:7, 9:10). When Proverbs uses the word "fear" here this is more of an understanding of power and greatness of God. It is to be in awe of something. An example could be to fear the ocean because you understand its power and unfathomable size. Yet you still are in awe of its beauty and power. The idea isn't to cower in terror, but to act with good sense and respect. Wise "fear of the Lord" means recognizing God is only true and trustworthy source of understanding about how life works best (Proverbs 8:32–36). In fact, He's the ultimate authority on how we ourselves work best. He knows us better than we know us.

People who have God's wisdom use His Word as the standard against which all other ideas are judged. If it disagrees with His revelation, it is wrong. Our hearts desperately want to find the answers ourselves, but we always need to come back to that ruling standard of God's wisdom in the Scriptures (Proverbs 4:10; Psalm 119:81; 1 Corinthians 4:6; 2 Timothy 3:16).