If it be asked why the Church should concern itself with “worldly affairs” and issues seemingly so far-removed from religion as economic theory, we can respond plainly that these two spheres are not really as distant from one another as the materialists and technocrats would have us believe. Yet a more comprehensive explanation is called for if we are to understand not only why the Church is justified in formulating this doctrine, but also why, once formulated, it deserves to be obeyed.
We may find such a justification by referring to the maxim of St. Thomas Aquinas, that grace presupposes nature.
That is to say, the spiritual dimension of man’s being, in which his true happiness is to be found and which is the supreme concern of the Church, is not to be imagined as existing in some other world, sharply divided from the “ordinary” world in which we live out our daily lives. The Christian tradition does not buy into such a dichotomy. It teaches that man is neither material body nor incorporeal soul, but is rather a union of body-and-soul. Grace, then, or the life of the spirit, presupposes and is built upon the foundation of nature. And while it is true that the foundation is hierarchically inferior to the superstructure (grace), it is still the foundation, and foundations are something of a necessity to the structures that rely on them. Taking this premise into account, the suggestion that spiritual authorities should not concern themselves with economic and political affairs and instead “stick to religion” is evidence that the speaker has made one of two great errors.
First, it is possible that he imagines that the higher order of reality (spiritual life) has nothing to do with the lower order (the physical world). From such a point of view, it is possible to conceive of a Church whose “sphere of competence” is religion alone, and whose business therefore has nothing to do with earthly life. This is probably the more common mistake. It is closely connected with the modern tendency already mentioned above, to try and organize (“systematize”) life into neatly divided categories. Unfortunately, given such a view of life, and because physical realities press themselves upon our senses incessantly, sooner or later the material concerns begin to claim most, if not all, of one’s attention. By going down this road, a man begins by dividing two orders of reality into separate worlds, and he ends by losing one of those worlds entirely as it fades from his consciousness. His point of view becomes an implicit, and sometimes also and explicit, materialism.
If we avoid the first mistake and manage to retain both orders of reality in our considerations, we must also guard against a second error, which comes from a misunderstanding of the hierarchical relationship between the spiritual and the material orders. In this case, even though the spiritual order is not lost, it is still hopelessly alienated from “worldly affairs” due to imagining the two orders as being “separate but equal,” when in fact they can only be comprehended hierarchically.
To understand the nature of this second error, consider two strangers who meet on the street. They are distinct and roughly equal—and for this reason it is improper for one to interfere with the business of the other. This is an appropriate view of two men on the street, but it is not an appropriate view of the relationship between the Church and the State, because this latter is one of hierarchy and not of equality. This type of relationship can be illustrated interpersonally by imagining a father and child. The father can and should interfere in the life of the child. And from the point of view of the child, the reverse is true: he cannot command the father, but instead should listen to what the father commands because he is by nature in a subordinate position.
Such is the nature of all hierarchical relationships. While the inferior cannot comprehend or inform the superior, the superior can always comprehend and should inform the inferior. And so, while the lower cannot transgress into the higher, the higher is in a legitimate position to guide the lower. Traditionally speaking, this is the proper view of the relationship between the Church and authorities of a strictly worldly order.
 ST I-II, q. 2, a. 1.