Crusius first came to notice as an opponent of the philosophy of Gottfried Leibniz and Christian Wolff from the standpoint of religious orthodoxy. He attacked it mainly on the grounds of the moral evils that must flow from any system of determinism, and attempted to vindicate the freedom of the will. The most important works of this period of his life are Entwurf der nothwendigen Vernunftwahrheiten (1745), and Weg zur Gewissheit und Zuverlässigkeit der menschlichen Erkenntniss (1747). Though rambling, and lacking originality, Crusius' philosophical books had a great but shortlived popularity. His criticism of Wolff influenced Immanuel Kant at the time when his system was forming; and his ethical doctrines "are mentioned" with respect in the Kritik of Practical Reason.
Crusius's later life was devoted to theology. He led the party in the university which became known as the "Crusianer" as opposed to the "Ernestianer," the followers of JA Ernesti. The two professors adopted opposite methods of exegesis. Ernesti wished to subject the Scripture in the same way as other ancient books; Crusius held firmly to orthodox ecclesiastical tradition.
There is a full notice of Crusius in Ersch and Gruber's Allgemeine Encyclopädie. See also JE Erdmann's History of Philosophy; A. Marquardt, Kant und Crusius; and article in Herzog-Hauck, Realencyklopädie (1898).