For Scripture authority St Thomas might have quoted: The Lord is high above all nations, and his glory above the heavens (Ps. CXII). God's free will is contained in His real physical distinctness from the universe, His sovereign pre-eminence over it, and absolute independence of it. Identify God with the universe, and logically He is no longer free. Pushed to extremity, Idealism means Pantheism, and Pantheism Determinism. For if God has no free will, still less has man.
Of God and His Creatures: 1.88