The phrase “separation of church and state” is not in the Constitution. It is, however, a useful way, as Jefferson implied, to interpret the First Amendment.
Consider, the actual first phrase of the amendment, “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.”
There is a detailed literature explicating this passage and its legal meanings. For my purposes in this column, it is only important to stress that the “making of law,” as understood by the Constitution as a whole, is a process that grounds the authority of the law in the “will of the people.”
Governments organized under the rule of a religious establishment, theocracies, ground such an authority in the “will of a deity.”
Government “by the people” does not appeal, as a final authority, to the will of such a deity. It is based upon a process of argumentation that depends upon information, experience and logic.
If it were based upon religious authority, America would be by definition a theocracy. And there would be no need to declare it so or officially recognize a State Religion.
Thus, if what we presently call “Christian Nationalism” (as a movement) is successful in placing its disciples into positions of leadership in the nation we would have a theocracy, in all but name. And it would violate the Constitution.
Put it another way. If you base your reasoning for a particular law on your interpretation (or your churches interpretation) of God’s will, you will be one voice advocating the establishment of religion (your religion as ultimate authority in matters of public policy). If a majority of you act this way, then, for as long as you control the government, you will have established your religion as the basis of lawful authority.
Given the historical reality that people, always it seems, differ as to their interpretations of God’s will, such a government would invoke a form of conflict that brooks few compromises and would almost invariably lead to suppression of opposing viewpoints.
Some Gods seem to expect no less.
Fortunately, for America, to this point, the Christian God of most of its religionists does not expect this. In fact, advises against it, advocates a separation of church and state.