Karl Friedrich August Kahnis (22 December 1814 - 20 June 1888) was a German Neo-Lutheran theologian.
From a poor background, Kahnis was educated at the gymnasium of his native town Greiz, and after acting as private tutor for several years began the study of theology at Halle. He was at first an ardent Hegelian, but he passed to orthodox Lutheranism. The transition may be dated from the publication of his Dr. Ruge und Hegel: Ein Beitrag zur Würdigung Hegelscher Tendenzen (Quedlinburg, 1838).
At the invitation of Hengstenberg, Kahnis went in 1840 to Berlin, where he studied under August Neander, Marheineke, Twesten, and others. To August Tholuck's Litterarischer Anzeiger für christliche Theologie he contributed a criticism of David Strauss, which appeared in expanded form under the title Die moderne Wissenschaft des Dr. Strauss und der Glaube unserer Kirche (Berlin, 1842). In 1842 he became privat-docent and then spent two years in close relationship with Neander, Henrik Steffens, and the circle of romanticists who gathered about Ludwig von Gerlach.
In 1844 he was called to Breslau as professor extraordinary to represent the orthodox party in a rationalistic faculty, but in his inaugural speech De Spiritus Sancti persona he departed from the accepted doctrine of Trinitarianism, ranking the Son as subordinate to the Father, and assigning the last place to the Holy Spirit, which he described as the impersonal principle of life, binding together the other two. Hampered by the lack of harmony between himself and his colleagues, he devoted himself to investigation in theology, the first results being his Lehre vom heiligen Geiste (Halle, 1847).