In this previously unpublished lecture delivered in Leysin’s Swiss University Sanatorium on September 15, 1943, Karl Barth presents Christian knowledge under the three aspects of its reality, possibility, and necessity. The aim is to briefly delimit the contours of this knowledge, via a constant attention toward its object, God’s word, as well as the human subject and his fallibility. In a final section, Karl Barth comments on the advantages and disadvantages he perceives in the two main currents which stand in contradiction within contemporary Protestantism, namely between liberal theology and orthodox theology.