The dogmatic constitution Dei Filius of the First Vatican Council held as a matter of faith that it is possible to prove the existence of God through the natural light of reason and apart from the aid of revelation. The doctrine has been criticized for its abstractness and lack of historical consciousness, in that it neglects the conditions in the human subject for the possibility of such a proof. Denys Turner has recently defended this claim of Dei Filius. In Faith, Reason and the Existence of God (Cambridge, 2004), however, Turner does not address the nuanced position of Bernard Lonergan, who interpreted Dei Filius in a way that defended its conclusion but severely limited its applicability. I propose to bring Turner and Lonergan into conversation on the matter of Dei Filius' doctrine regarding the possibility of proving the existence of God.