After his ordination in 1896, he served as a pastor for the Protestant community in Cairo (1896), in Burg, Stein am Rhein (1899) and then in Geneva (1904), where he met and befriended Karl Barth as his vicar, and finally at St Peter's parish church in Zurich.

He was a friend of Sigmund Freud,[2] Carl Jung, Thomas Mann, and Albert Schweitzer – and thus was influenced by the spiritual tendencies of the twentieth century.

[3] Keller was one of the first pastors to become interested in psychoanalysis and met Jung in 1907, and later at the fourth psychoanalytical congress in Munich 1912, where he witnessed the break between Sigmund Freud and C.G.

In addition, he served until 1945 as Secretary-General of the European Central Office for Ecclesiastical Aid, founded in 1922, and dedicated to the Europe and Russian, Armenian, Assyrian-born and "non-Aryan" refugees.

In addition to publications on the ecumenical movement, it also includes an introduction to the philosophy of Henri Bergson, contributions to the relationship between psychoanalysis and Christianity and several volumes in which "secular devotions" are collected.