What did Les mean by saying the Beatitudes are the Constitution of the Kingdom?
The Beatitudes serve as the Kingdom's governing charter because they match Isaiah 11's description of Messiah's righteous rule—His Spirit-given wisdom, understanding, counsel, might, knowledge, and fear of the LORD, and His equity for the meek and poor. They express the righteous principles by which Christ will judge and rule.
Les teaches that Isaiah 11's prophesied Branch from Jesse, the Messiah, will rule not by sight or hearing but with righteousness, judging the poor and reproving with equity for the meek. Those words, Les says, remind him of the Beatitudes in the New Testament, so the Beatitudes become the very Constitution of the Kingdom—the foundational standards of Christ's coming reign. The passage also shows the Spirit of the Lord resting on Him with wisdom, understanding, counsel, might, knowledge, and fear of the LORD, and His rod and breath slaying the wicked at Armageddon.
And shall make him of quick understanding in the fear of the LORD: and he shall not judge after the sight of his eyes, neither reprove after the hearing of his ears:
But with righteousness shall he judge the poor, and reprove with equity for the meek of the earth: and he shall smite the earth with the rod of his mouth, and with the breath of his lips shall he slay the wicked.