This essay examines the role of Jose Miguez Bonino as a Latin American Protestant observer at both Vatican II and the CELAM Conference in Medellin (1968), as well as explores his impact on the solidification of Latin American ecumenism, particularly through his critical engagement of the "ecumenical establishment" and his emphasis on the "social question" in addressing overlaps and tensions between Christian unity and social justice. It ultimately argues that Bonino contributed to the rise of an ecumenical liberation Christianity and a third- world ecumenical perspective that continues to be significant for ecumenical practice and theology today.