Lydia McGrew argues that the problem of evil poses a substantial problem for natural theology. If we take, say, the famous trilemma of pseudo- Epicurus
If God is willing to stop evil but unable, how is he omnipotent?
If he is able but not willing, how benevolent?
If willing and able, how is there evil?
we can read it as showing that Christian and natural theology come to different conclusions about whether God is benevolent or indifferent with respect to the world. It seems that natural theologians are forced to conclude that God is indifferent, and that they must confess frankly that they stand in need of revelation in order to establish the crucial fact of Christian theology that God loves the world and desires to give good things to human beings. While she initially poses the problem with respect to the problem of evil, a more forceful way of putting McGrew’s argument (which is implied in how she develops the point in the comments) is this:
In order to defend God’s benevolence in the face of evil, one must be told God’s plan to remedy evil, but
God’s plan to remedy evil belongs to the history of salvation.
Salvation history is known only by revelation, therefore, etc.
The argument admits of more than one interpretation, and I need some groundwork in order to respond. The argument is a general argument about what reason can know purely by itself, and I think this question is usually understood poorly, and needs to be visualized in a better way.
St. Thomas argues that God needed to reveal even things that can be known by natural reason, since the truths which can be known about God were known only by few, after a long time, and with much error. It’s crucial to notice that this state continues throughout all time. Natural theology or metaphysics are the most difficult of sciences. Almost no one can learn them at all, and of those who can, none can learn them quickly and all commit many errors. The sense we have that we can attain certainty early and easy is not entirely illusory, but this certitude depends on our practicing metaphysics in a safe environment that doesn’t expose us to the full force of all possible objections from the beginning. Example: a flower kept in a greenhouse will really survive all the shocks and stresses it encounters, and in this sense it survives “by its own nature alone”, but it is not exactly in its natural place, and it wouldn’t survive at all if exposed to all possible stresses or strains we could inflict on it. The power of “pure reason” is like the survival power of a flower: it needs an environment that not only protects it not only from what it might encounter in it natural environment, but also from things that, while not part of it natural environment, might be introduced to kill it off (like pesticide). Institutions – normally churches and institutions affiliated with them - provide this environment. The proof of that the environment is the correct grows as the person nurtured in it finds himself able to assimilate more and more truth, and overcome more and more objections.
Even after metaphysics becomes strong enough to live on its own, there is another difficulty in that its truth is found only in the soul of the one studying it. There is no product that the metaphysician can produce to prove the truth of his efforts and no planes crash if he is wrong. The metaphysician is seeking just truth and knowledge, and both are interior perfections that no one experiences except the one who has them.
The question of what we can determine by pure reason is set against this backdrop. If we understand the question of what reason can know by itself to mean “what can a reasonable person expect to know about metaphysics if he tries real hard to figure it out for himself”? then the answer is “almost nothing”. A person might present this “almost nothing” in a dogmatic or bull-headed way, or bully their way through objections, but this doesn’t change that they know.
Taken in this way, what is proper to McGrew’s question drops off and we see it in light of the general difficulty of all questions in metaphysics or natural theology. All natural theology is hard, it requires exterior help, and when pure reason finally succeeds fully, it is only within the soul of the one knowing, who is aware of how arbitrary his doctrine must look to anyone who has it presented to them in their natural habitat, or (what is quite common nowadays) from the pesticides that we introduce artificially to kill off even those plants that might have survived in the wild.