Modern humanism, in the form of Marxism and existentialism, is humanism without God where the present-day man has lost his transcendental sources. These novel, "post-modern" philosophies have created a new mythology, i.e. the myth of existence (Sein) and ontology. In the 20th century, ontology is no more the science of the existent but that of the existence. According to Heidegger, all previous philosophy forgot about existence, beclouded being. In my essay I aim to show that the accusation of forgetting about being is valid exactly for the new, fashionable philosophies. They constantly refer to "existence", whereas in fact they narrow it down to material and perceptible existent (the matter) or the subjective existent (the human being), i.e., they are concerned with the existent and not existence itself. And this is the root of the tragedy of modern man. With regard to Christian tradition, and thus Thomism-neo-Thomism, ontology had indeed been the science of the existent (ens). However, an in-depth analysis shows that existence (esse) and not the existent stands at the core of St. Thomas's teachings. Hence the accusation of Heidegger is not valid for Aquinas. As a matter of fact, St. Thomas elaborated an extraordinary ontology even in a "post-Heideggerian" sense. St. Thomas himself is going to be our guide in this treatise. We might as well say that Aquinas himself answers the problems of existence of present-day man. One can build an undistorted humanism upon this ontology, which avoids the one-sidedness and immanentism of both materialism and subjectivism. The Christian humanism of the man open to existence, to God, is the unspoilt heritage of Saint Thomas.