This feels like it could be a reference to the Kabbalah, particularly to the top (Keter) and bottom (Malkhut) sefirot which are identified respectively with the Crown and the Kingship/Rulership of God.

Looks like what Milton is trying to justify is that everything happened according to God's plan, including the fall of man from paradise and the coming of Jesus to save all. Allusions to God's providence and nothing happening without his will are present throughout the Bible, particularly in the book of Job where God gives Satan explicit permission to test Job's faith.