This project that is made possible by a generous grant from the John Templeton Foundation (Grant#60806) continues data and results from past Bielefeld-Chattanooga research projects into a longitudinal study. The faith development interviews and questionnaire data collected in this current project constitute Wave III of a longitudinal study with first-time participants in the Deconversion Project and the Spirituality Project (which together are considered Wave I) and with first and second participations in the study “Developmental Change in Spirituality” (Wave II, completed 2017). The current and all past projects are multi-method in design, since they are based on (a) faith development interviews (FDI) and (b) questionnaire participations. While the number of questionnaire data is considerably higher than the number of FDIs, the cases with both the FDI and questionnaire responses are the most valuable for investigating religious development. Therefore, such longitudinal cases are the focus of this preregistration. Data collection has already begun, but is far from being completed: We aim at September 2019 to have received most of the responses to our online-questionnaire. But FDIs need not only to be conducted and then fully transcribed, but FDIs require time-consuming evaluation, before results can be entered into an SPSS data base.