Natural religion

William G. T. Shedd begins his introduction to Augustine, On The Trinity by claiming that:

The doctrine of the Divine Unity is a truth of natural religion: the doctrine of the Trinity is a truth of revealed religion.

This might seem obvious to Shedd (who wrote in 1887) but there are still plenty of atheists and polytheists in the world who might dispute it. Maybe the idea of many competing gods comes pretty naturally to humanity.

And indeed we might argue that there are traces of the Trinity, or at least of the one and the many, unity and differentiation, in the world (and perhaps especially in the image-bearers) that the Triune God has made.