"Spirit," referring to the essential force within God, is used to help us understand the attitude motivating His actions. What we see on the outside, externally, is judgment; but spirit is an internal, invisible force, impelling, motivating, generating His judgment, which is "burning," representing a punishing, purging, purifying destruction. So in God, His spirit motivates Him to judge. This is how "spirit" is used in God's Word.
John W. Ritenbaugh
The Holy Spirit and the Trinity (Part Two)
He is called "the Branch of the Lord"—He is God's descendant! He is the Son of God, not just the son of the man David! Yet this verse also describes Him as "the fruit of the earth," meaning that, though He is God, He is also from mankind. He could claim full descent from both Godkind and humankind!
Richard T. Ritenbaugh
Other Forerunner Commentary entries containing Isaiah 4:2: