James Barr believed there was a Jewish background to natural theology. This made it likely that the passages in the New Testament that seem to imply natural theology actually do imply natural theology. This is my understanding of his argument so far in Biblical Faith and Natural Theology.
Part of his argument is that Paul has strong affinities with the Wisdom of Solomon, a Jewish work in Greek that is part of the apocrypha or deutero-canonical Bible recognized by Catholic and the Eastern Orthodox Christians, but not by Protestants.
One of the affinities is that both Paul and Wisdom of Solomon have a doctrine of the Fall centered around Adam’s sin. In the Hebrew Bible Adam’s sin is not a hugely important incident and there is no idea that before this there was no death. However, both Paul and the Wisdom of Solomon see Adam’s sin as determinative for human sinfulness and mortality.
This is important partly because Karl Barth based much of his opposition to natural theology on the fallen state of man. Barth had said that without the Fall, natural theology would be possible. But the Wisdom of Solomon and some of the Enoch literature are the only pre-Christian Jewish sources for this kind of notion of the Fall. It is an idea outside the mainstream of Judaism.
But that is a supplementary point. The main affinity is between the 13th chapter of Wisdom of Solomon and Paul’s argument in Romans 1 and 2.
To sum up this point then both Paul and Wisdom begin from creation and provide an account of the way in which people failing to recognize the reality of the creator God entered into idolatry and thence into the vilest immorality. Both say that God the true God was known to people which is why their idolatry was disgraceful and inexcusable. Both hold it clear that God is knowable through the things that he has made.
So natural theology came into Christianity from Judaism. Barr qualified this point by saying that both the Jewish background and Paul’s connection of human knowledge to creation was not based on the structure of creation, but upon human constructions that depended on historical context. Apparently Barr unpacks this idea later. For now, I understand him to say that natural knowledge of God is not as obvious as some make it out to be. But in answer to certain problems or conflicts natural theology makes a useful contribution.
Barr seems to have thought that Paul probably got his argument from reading the Wisdom of Solomon. But he did not hang much on that. Paul might have been drawing on the tradition that also lies behind Wisdom. Direct dependence is not crucial to Barr’s case.
Barr said that it was the Jewish religion itself that made the adoption of certain Greek philosophical categories attractive. Chief among these was the category of nature. It helped the Jews to understand their own law and wisdom as in accordance with nature. Philo, in particular used the category of nature to support the Jewish law. Barr acknowledged a great gap between the thought of Paul and that of Philo. But, he said that in the particular case of connecting creation with nature they were similar.
A small point that interested me was the background in the Egyptian diaspora of some of these ideas. Except for the golden calf, the animal representations of pagan idols gets little attention in the Hebrew Bible. Egyptian religion, however, was full of iconic representations of animals. When many Jews began to live in Egypt after the Babylonian exile and during Hellenistic times, they reacted against this. So the attack on idolatry in the Wisdom of Solomon shows horror at the use of even disgusting animals as divine representations. So we also have Romans 1:23:
And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things (KJV).
I found the following image that is supposed to show Egyptian idols, especially those carried in public processions: