A theory of professional action is developed based on the 3 modes of inference: abduction, induction, and deduction. The theory explains how professionals reflect in action by combining a single case to their professional repertoire. With teachers in religious education (RE), the problem is focused on how they link students' new and individual religious semantics to traditional Christianity to make religious communication possible. Therefore, the professional-teacher action in religious education starts with the religious change in a plural society. Next professionals react to this change by means of the modes of inference. As part of this idea, a theory of inference-based professionalism is created, which relates Schon's theory of reflection-in-action to the modes of inference. The theory is grounded or based on qualitative empirical research. Third, research design and its results are presented and, finally, an outlook on transformative professionalism concerning planning, carrying out, and evaluating religious education-lessons is discussed. (Contains 4 tables and 2 figures.)