Gustav Adolf Deissmann (7 November 1866 – 5 April 1937) was a German Protestant theologian, best known for his leading work on the Greek language used in the New Testament, which he showed was the koine, or commonly used tongue of the Hellenistic world of that time.

Members included Ernst Troeltsch, Max Weber, Eberhard Gothein, Georg Jellinek, Karl Rathgen, and Wilhelm Windelband.

[3] In Berlin, Deissmann's academic focus began to shift from Greek philology to the ecumenical movement, church reform and, significantly, international Völkerverständigung (i.e. peace-promoting mutual understanding between nations and cultures).

Its target audience was primarily influential German and American Christians, and it provided a forum for the advancement of peace and understanding among nations.

Deissmann campaigned single-handedly for several years, both on a national and international level, to raise awareness of the plight of Ephesus, and managed to organise funding for the archaeological work to recommence in 1926, and continued annually until 1929.