The study is an introduction to Oswald Bayer’s groundbreaking work Promissio: The Reformational Turn in Luther’s Theology from 1972, which is now also available in English. It analyzes the research situation of the book and the approach and implementation of Bayer’s investigation, which distinguishes between Luther’s pre-reformational theology (until 1517) and his reformational theology since 1518, substantiating this distinction with a large number of meticulous textual analyses. The key observations and arguments are highlighted and compared with recent research and in some cases corrected. Particular emphasis is placed on the fact that linguistically identical formulations and sentences can have different meanings in early and later contexts. While for Bayer the “reformational turn” meant an insurmountable separation from Catholic doctrine, the study argues that Luther’s discovery of the sacramental character of the promissio (promise) is precisely the basis for a promising Catholic-Lutheran ecumenism. Bayer’s findings are also shown to be fruitful for the interdisciplinary discourse on “gift giving.”