The first deals with those truths about God which Aquinas believes to lie within the reach of human reason – such as the existence of God, and the way in which God can be seen in the natural world. The second part deals with core Christian truths that lie beyond the reach of reason, such as the concepts of incarna- tion and the Trinity. “Some things that are true about God lies beyond the competence of human reason, such as that God is Three and One. Yet there are other things to which human reason can attain, such as the existence and unity of God, which philosophers have demonstrated to be true under the guidance of the light of natural reason.” Aquinas’s agenda appears to be to lead his readers into the more complex world of Christian truths that lie beyond reason by beginning with the less challenging truths that are accessible to reason.
Yet in the end, perhaps the most important feature of this work is its basic conviction, rather than the specific way in which this is explored – namely, that the Christian faith is eminently reasonable. Where it goes beyond reason, Aquinas argues, this is to be seen as transcending the limits of reason, not contradicting reason. Revelation brings reason to completion. Since both reason and revelation seek the same basic truth, Aquinas argues, they are to be seen as complementary rather than contradictory in their relationship. Although this view can be found throughout Aquinas’s writings, it is seen at its clearest in the Summa contra Gentiles.