Given the background we looked at in yesterday's posting, one possible interpretation of Quas Primas is that Christ rules as "king" (an equivocal term, as His response to Pilate made obvious: "Are you a king?" "That is your word for it.") through our personal adherence to the precepts of the natural law and by conforming the laws and institutions of the nation (as well as our personal behavior) as far as possible to them.
This makes sense if we stop to think about it. Within the Aristotelian/Thomist framework, the natural law is based on God's Nature, self-realized in His Intellect. You might not be able to tell much about God, but if you accept that God made man in "His image and likeness," you can conclude that what the general consensus of humanity accepts as good and in conformity with human nature must be good and in conformity with God's Nature.
God is therefore the "personification" of the natural law; i.e., God doesn't simply do good, He is good. If we believe Jesus to be (as the Creed has it) "consubstantial with the Father," i.e., part of one God in three divine Persons, then Jesus — being as fully God as God-the-Father — is also the personification of the natural law.
That being the case, Christ "rules" as "king" of the universe by providing us the perfect example of compliance with human nature, and thus the Divine Nature with which He is "consubstantial." Christ is king and lawmaker not because He passes laws, but because He is law itself. What human courts and governments can say only metaphorically (and correctly only when they adhere to the precepts of the natural law), Christ says as a simple statement of fact: "I am the Way, the Truth, and the Life." Christ rules humanity as king when we adhere to the precepts of the natural law, the most basic of which is that good is to be done, evil avoided.
The principal way in which we conform ourselves to human nature is by exercising our rights in such a way as to acquire and develop virtue. In society, the most important rights — and thus the most important means of acquiring and developing virtue — are life, liberty (freedom of association/contract) and . . . property.
Why? Because (as Daniel Webster observed) "power naturally and necessarily follows property." "Power" is defined as "the ability for doing." Thus if you don't have property, you can't "do." If you can't "do," then you can't acquire and develop virtue, so what's the point of society if nobody owns, as in socialism, or only a few own, as in capitalism?
Does this mean, however, that non-Christians, or even non-Catholics have no place in what Pius XI called "The Reign of Christ the King," and described as "the peace of Christ in the Kingdom of Christ"? Nothing could be further from the truth.
Given that 1) Christ (with the Father and the Holy Spirit) is the personification of the natural law, 2) He rules the universe through our compliance with the natural law, 3) the natural law is written in the hearts of all men (and women and children if you insist on inclusive language), then anyone who conforms him- or herself to the precepts of the natural law as far as he or she is honestly able is being ruled by Christ as king. You don't have to accept Jesus as Savior or even as God or even, frankly, ever have heard about Him in order to be subject to His rule and obey Him.
We are not talking here about whether non-Christians can be saved, but what is the proper basis for civil government in the temporal order. Pius XI was explaining that the basis for a just civil society is the natural law as that term is understood by the Catholic Church. Given that, even an officially atheist State, as long as it recognized and protected the natural rights of its citizens (including freedom of religion), would in that sense be ruled by Christ the King.
Rulers must conform materially to the natural law, or they do not rule justly and can be replaced, or even the form of government changed. Aquinas said pretty much the same thing in De Regimine Principum ("On the Governance of Rulers").