When modern philosophers treat of religious matters, they almost always regard an incoherent human theory as if it represented Christian faith. Hobbes, Hume, Spinoza, Kant, Hegel, Feuerbach, Marx, Nietzsche, Dewey, Sartre, and many others deny divine revelation.⁴⁹ Yet they retain a residue of what Christians believe, especially in their exalted idea of the human person or, at least, in the ideal by which they criticize human life. Since it is the business of philosophers to articulate their beliefs, to show the reasonableness of holding them, and to answer objections from all sides against them, modern philosophers such as those named have articulated more or less tightly integrated atheistic systems (see GS 19–21).

Modern philosophical systems are plausible, complex, and seemingly powerful. Yet they are ultimately inconsistent. Hume, for example, holds that reality necessarily excludes anything which is not contingent—that is, which is necessary; in this way he tries to exclude God. Kant holds that human knowledge cannot extend to God because it is limited to the world of experience; he thinks he shows this by his theory of the sources of knowledge, yet he locates these sources outside experience. Hegel maintains that one cannot know anything in a fully true way short of knowing the totality of reality; he believes that at this point knowing and what is known coincide; yet, at the same time, Hegel thinks his own philosophy is true knowledge and that he can exclude other philosophies as false. The post-Hegelian philosophers mentioned above and many others hold that the complex of human thought and action is the ultimate source of all meaning and value; at the same time, they try to exclude as illegitimate the belief and way of life of Christians, since we do not agree with their view that God is to be replaced by the human mind—and replaced in the particular way each atheistic humanist personally prefers.

The preceding paragraph is not intended as a summary refutation of the leading approaches in modern philosophy. I have treated these matters at length in a previous work.⁵⁰ Rather, my intention is to make it clear that Catholic theology must be very careful in borrowing from modern philosophies and from theologies which have been shaped by modern philosophies. Vatican II continues to commend St. Thomas as a guide for Catholic theological speculation (see OT 16). The Holy See continues to point out that modern philosophies are not the apt instruments for Christian reflection which ancient philosophy was for the work of Thomas.⁵¹