For several decades, the traditionally Catholic region of Flanders in Belgium has been subject to processes of secularisation. However, whereas in the past individuals' decision to no longer attend church was typically a passive or silent one, in 2023 it became a subject of vocal debate. Following the screening of a documentary that exposed the extent and impact of sexual abuse within the Church in Flanders, almost 15,000 individuals formally left the Church. They went to the trouble of sending a letter to the diocese in which they were baptised, asking to be officially removed from the baptismal records. In Annual Reports published since 2018, the Catholic Church in Belgium self reports on its 'state'. These reports highlight the pastoral activities, number of baptisms, money spent on caritas, to name but a few of the key aspects. Among a variety of numbers, it mentions the number of people that formally left the Church. The present paper will present these figures and contextualize them within the broader context of the recent sociological numbers since the 1980s, with a view to drawing out fundamental theological questions. This suggests that the structure of the paper will be divided into three sections. Firstly, the recent numbers as presented by the Church will be examined, contextualised, and their validity will be ascertained to the extent that it is possible. By framing these figures within a broader set of data on church practice in Flanders, in a second part, I will draw out theological questions about how the Catholic Church thinks about membership, defection, and deconversion, because Catholics are still obliged to maintain communion with the Church. Finally, the paper suggests that further quantitative and qualitative research is urgently needed to gain a better understanding of people's motivations for staying in or leaving the Catholic Church.