Cosmopolitism as a threat to national life from the point of view of the social teaching of the Church. The sources used here are the pronouncements of the following Popes: Pius XII, John XXIII, Paul VI and John Paul II. The social teaching of the Church underlines erroneous ontological and ethical presuppositions that underlie cosmopolitism. Those presuppositions lead to the false understanding of the existence of a nation and to apotheosis of freedom of an individual. It precludes a proper determination of the obligations toward the national community. In the light of Magisterium Ecclesiae, cosmopolitism, understood as underestimation and rejection of the national values, harms man in the final analysis, for it makes the full growth of man impossible, and it obscures fundamental need for social identity. Thus, it threatens the peaceful coexistence in the family of nations.