The article intends to show how the insistence of Pope Francis on the centrality of the poor and marginalized and of suffering humanity in general is not something coincidental or secondary in the Church, but something constitutive and determinant of its identity. It is a fundamental dimension and truth of Christian faith. This is a question as much of ecclesial orthopraxy as of theological orthodoxy. And for this reason it ought to be taken as a fundamental mark of the Church. For this, we will outline the problematic of the mystery of the Church and its formulation in terms of ecclesiological marks and demonstrate in which sense being of the poor constitutes a fundamental ecclesiological mark. It is as foundational as the classical marks that appear in the Nicene-Constantinopolitan formula (one, holy, catholic, apostolic), although its formulation in dogmatic terms may be recent and in need of major development and elaboration.