When Jesus refers to the kingdom of God he is referring to the final form of God’s kingdom, which is the saving kingdom that he ushers into the world through his perfect life, atoning death, and glorious resurrection. That is to say: Jesus is not referring with the phrase “kingdom of God” to God’s universal kingship over all things in a generic sense, but to the kingdom that will be manifest in the salvation he accomplishes and then pours out on his people. It is a “spiritual” kingdom, though it has profound implications for how its citizens live in this world.

A Wall of Separation

Thomas Jefferson famously wrote in an 1802 letter to the Danbury Baptist Association that the First Amendment to the U.S. Constitution prohibiting the Federal government from making any laws “respecting an establishment of religion” meant that there was, and must be in America, an absolute “wall of separation between Church & State.”

Although Jefferson was only referring to establishment on the Federal level, which is indeed prohibited in the First Amendment, his phrase has come to represent for many Americans something much more expansive. It has, in fact, become a commonplace to indicate that the State can have nothing whatsoever to do with God or even the basic moral truths found in the Bible. Such an understanding has become predominant even among many Christians. But is it correct?

Separating church and state is extremely important. It is thoroughly biblical to do so, and the best thinkers in the Christian tradition have recognized the importance of doing so, although in a way very different from the modern conception of Jefferson’s wall of separation. There is a sense in which church and state must be absolutely separate and a sense in which they cannot be thought of separately at all. Each has its own unique realm of authority that must be preserved from unwarranted intrusion from the other, while neither can be sealed off completely from the other.

However, to adequately address the relationship between church and state we have to back up. The broader historical-theological concept into which the discussion of church and state falls is that of God’s “two kingdoms.” At its most basic level, the classic Protestant two kingdoms doctrine means that God rules over his spiritual kingdom, the church, in one way, and rules over the world outside the church in a different way. This is sometimes taken (wrongly) almost as if God doesn’t rule over the world outside the church at all, but it should not be understood in that way.

In this article I will introduce the doctrine of God’s two kingdoms, and then I will more briefly focus on how this idea illuminates the relationship between church and state. I’ll also explain some key biblical texts that deal with these difficult (and often fraught) relationships. The goal is to help Christians understand the divine purposes for each realm.

Defining the Two Kingdoms

As Brad Littlejohn puts it, for classical Protestant thinkers: “The two kingdoms were not two institutions or even two domains of the world, but two ways in which the kingship of Christ made itself felt in the life of each and every believer.” Referring to Christ’s comprehensive reign over all things Abraham Kuyper famously wrote that “there is not a square inch in the whole domain of our human existence over which Christ, who is Sovereign over all, does not cry: ‘Mine!’” All fine and good, but what does this mean precisely? For example, if Jesus is king over all things should church and state be merged, with the officers of the church ruling the state as well? Should the state rule over the church? Is there another way that such realms should be related? The classic Protestant doctrine of the two kingdoms helps answer these very questions.

This is not an exhaustive historical survey, so I’ll simply quote from John Calvin to illustrate this historical strand of thought:

The former [the spiritual government] has its seat within the soul, the latter [the temporal government] only regulates the external conduct. We may call the one the spiritual, the other the civil kingdom. Now, these two, as we have divided them, are always to be viewed apart from each other. When the one is considered, we should call off our minds, and not allow them to think of the other. For there exists in man a kind of two worlds, over which different kings and different laws can preside . . . . The question . . . though not very obscure, or perplexing in itself, occasions difficulty to many, because they do not distinguish with sufficient accuracy between what is called the external forum, and the forum of conscience.

Though it is sometimes mistakenly taken as such, Calvin’s point (which is representative of classic Protestant thinking on the whole) is not that there is one realm in which Christ rules (the spiritual realm) and another with which he has nothing to do (a non-spiritual realm), but rather that the Christian always lives simultaneously in both worlds. And it is also the case that Christ rules over both worlds, though his rule looks different according to the specific nature of each realm (for the sake of clarity and consistency I will refer to the “spiritual” and “external” kingdoms in the rest of this article).

Christ rules over the spiritual realm, or kingdom, by his word. In this kingdom the consciences of believers may only be bound insofar as Scripture itself binds them, and the focus of this kingdom is eternal salvation and the spiritual well-being of the saints. The spiritual kingdom is the sum total of believers and their children.

Does this mean the external realm, or kingdom, is a moral free for all? Not at all. Christ also rules over that realm, although in a fundamentally different way. The charter of the external kingdom is not the Bible (strictly speaking) though the Bible informs life in the civil kingdom. The charter for the external kingdom is derived in different ways from the imprint of God’s law in nature, the human conscience, the voice of tradition, human law and history, and more.

Properly separating the spiritual kingdom from the external kingdom that encompasses everything outside of the spiritual is vital. The spiritual kingdom, God’s saving work in the lives of his people, must be distinguished from everything earthly and temporal. Distinguishing, however, is not the same thing as radically separating or divorcing. My leaf blower’s engine requires a precise blend of oil and gasoline to operate. Oil is not gasoline; they are distinguished. But my engine will not run without both; they cannot be radically separated. The same is true of God’s two kingdoms.

The Two Kingdoms in Scripture

So far I’ve only been giving definitions and explanations. Now we must turn to Scripture. The focus in this section will be on a variety of texts that show us the distinction between God’s two kingdoms.

The Spiritual Kingdom

God is king over all things. Of this there is no dispute: “The Lord is king forever and ever; the nations perish from his land” (Ps 10:16); “For God is the King of all the earth; sing praises with a psalm” (Ps 47:7)!

In Jesus’s earthly ministry he also proclaims his Father’s dominion over all things, for example, teaching his disciples to pray for God’s kingdom to come, and his will be done, on earth as it is in heaven (Matt 6:10). But something unique and vital is introduced into Christ’s preaching of God’s kingdom.

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