Because the State exists to assist man in realizing the potentialities of his nature, and because his vocation is in its noblest sense a spiritual one, then political society fails automatically if it does not take into consideration anything more than the temporal lives of its citizens. As it has been put by Aquinas:
“[T]he same judgment is to be formed about the end of society as a whole as about the end of one man…If such an ultimate end either of an individual man or a multitude were a corporeal one, namely, life and health of body, to govern would then be a physician’s charge. If that ultimate end were an abundance of wealth, then knowledge of economics would have the last word in the community’s government. If the good of the knowledge of truth were of such a kind that the multitude might attain to it, the king would have to be a teacher. It is, however, clear that the end of a multitude gathered together is to live virtuously. For men form a group for the purpose of living well together, a thing which the individual man living alone could not attain, and good life is virtuous life. Therefore, virtuous life is the end for which men gather together…Yet through virtuous living man is further ordained to a higher end, which consists in the enjoyment of God…Consequently, since society must have the same end as the individual man, it is not the ultimate end of an assembled multitude to live virtuously, but through virtuous living to attain to the possession of God.”
Now these words should not be interpreted as a demand for State-run churches, mandatory attendance of the mass, and the like. Aquinas is merely acknowledging the fact that the State, since it plays the governing role in society, cannot make pretenses at spiritual indifference. Let it try to adopt the stance of indifferentism, and it will end by establishing a practical atheism.
So what might Aquinas’s vision look like in practice? To take but one example, the Church calls on the State to ensure that workers have a sufficient amount of rest, not merely to repair the strain placed on the body during labor, but so that workers can properly devote themselves to their spiritual exercises which can easily fall into neglect. Man must therefore be provided with rest for both soul and body, and not for the body alone. By such simple measures we can see how the State ought to act in favor of the religious life without assuming responsibility for it.
 CSDC, 386.
 DR, 106-107.
 RN, 41-42.