Christian August Crusius (January 10, 1715 - October 18, 1775) was a German philosopher and theologian.

He was born at Leuna near Merseburg in Saxony. He was educated at the University of Leipzig, and became professor of theology there in 1750, and principal in 1773.

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.

Crusius's chief theological works are Hypomnemata ad theologiam propheticam (1764-1778), and Kurzer Entwurf den Moraltheologie (1772-1773). He sets his face against innovation in such matters as the accepted authorship of canonical writings, verbal inspiration, and the treatment of persons and events in the Old Testament as types of the New. His views have influenced later evangelical students of the Old Testament, such as EW Hengstenberg and F Deutzsch.

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 art, in Herzog-Hauck, Realencyklopädie (1898).