Since the late 17th century, the Mekong Delta has become a vibrant place for Vietnamese, ethnic Khmer, and ethnic Chinese settlements, forming mixed communities in terms of race and culture. As a new frontier, the Delta has continuously undergone state-sponsored “civilizing” processes. The resulting hybridity in folk culture gave birth to various religious sects, especially on the Vietnamese-Cambodian border. From the view of late imperial Confucianism, these groups were judged as “heretical sects” and their leaders “heretical masters”. Despite being classified as “heretical”, these local “masters” and their communities still insisted on the core Confucian values of benevolence, righteousness and patriotism. Chinese secret societies and rebels escaped to the Delta in different times, bringing new ideas to the region, especially the cults of Five Lord Buddhas, Maitreya, and the concept of the birth of the wise king. Under French suppression, these “heretical masters” changed their religious strategies, supported local military leaders and secret societies to “fight the French and restore Đại Nam”. This research aims to investigate and analyse the “heretical” religious movements and ideological transformation of “heretical masters” in the Mekong River Delta in the late 19th and early 20th centuries, thereby strengthening the argument that although Vietnam’s local religious elites were marginalized and suppressed by state governance (the Nguyễn dynasty, French colonialists), they always cared about national independence and social prosperity, and therefore transformed their sectarian strategies into a special form of religio-political hybridity. National independence was definitely prioritized, and therefore the concept of “loyalty” in the traditional Confucian view turned into “patriotism” in the new context of French invasion and colonialization. This study uses a mixture of extensive field data collected between 2015‒2017 and 2022‒2024 (the primary data source for this study) with limited original texts from the Nguyễn dynasty and the research findings of selective previous scholars, and applies the theoretical concept of “standardization” and/or “orthopraxy” from the pre-modern Confucian tradition to examine and analyse political-religious practices, ideological transformation, and implicit narratives among local religious masters in rural Southern Vietnam, confirming that the Mekong Delta has fostered an environment of cultural and religious dynamism and pluralism since the late 19th century.