He well understands the concept until he gets to the Ten Commandments.
Justice Scalia, in his dissent in MCCREARY implies that "monotheism" has some type of constitutional privilege over non-monotheistic religions, at least in the context of government endorsement of monotheistic, over non-monotheistic religions. In that opinion he writes:
Besides appealing to the demonstrably false principle that the government cannot favor religion over irreligion, today's opinion suggests that the posting of the Ten Commandments violates the principle that the government cannot favor one religion over another....That is indeed a valid principle where public aid or assistance to religion is concerned...or where the free exercise of religion is at issue...but it necessarily applies in a more limited sense to public acknowledgment of the Creator.
If religion in the public forum had to be entirely nondenominational, there could be no religion in the public forum at all. One cannot say the word "God," or "the Almighty," one cannot offer public supplication or thanksgiving, without contradicting the beliefs of some people that there are many gods, or that God or the gods pay no attention to human affairs. With respect to public acknowledgment of religious belief, it is entirely clear from our Nation's historical practices that the Establishment Clause permits this disregard of polytheists and believers in unconcerned deities, just as it permits the disregard of devout atheists. The Thanksgiving Proclamation issued by George Washington at the instance of the First Congress was scrupulously nondenominational, but it was monotheistic.
Let me try to explain what I think is going on in Scalia's head. He is willing to entertain the notion that the Establishment Clause does more than forbid a national Church, that government may indeed be forbidden from favoring one sect over another in its mere acknowledgments, and he is looking to the historical record for evidence. What he finds is that all of the first four Presidents, like the Declaration of Independence, commonly invoked God in their public pronouncements. But he also finds that their invocations were "scrupulously nondenominational," so much so that they hardly can be termed "Christian" or even "Judeo-Christian." As Scalia notes, "This is not necessarily the Christian God (though if it were, one would expect Christ regularly to be invoked, which He is not)" and,
All of the actions of Washington and the First Congress upon which I have relied, virtually all Thanksgiving Proclamations throughout our history, and all the other examples of our Government's favoring religion that I have cited, have invoked God, but not Jesus Christ.
Scalia instead dubs him "the God of monotheism." And further notes, "[t]he three most popular religions in the United States, Christianity, Judaism, and Islam -- which combined account for 97.7% of all believers -- are monotheistic."
So Scalia doesn't really answer whether it is constitutional for government to endorse in its mere acknowledgments, one Christian sect over another, Christianity over Judaism, or Christianity and Judaism over Islam...but instead he concludes, based on the historical practice of the first four Presidents, it is constitutional to endorse "monotheism" over "non-monotheism." Monotheism therefore is a Lowest Common Denominator among Judaism, Christianity, and Islam.
With respect to public acknowledgment of religious belief, it is entirely clear from our Nation's historical practices that the Establishment Clause permits this disregard of polytheists and believers in unconcerned deities, just as it permits the disregard of devout atheists.
Scalia then connects such "monotheism" with the Ten Commandments themselves.
All of them, moreover (Islam included), believe that the Ten Commandments were given by God to Moses, and are divine prescriptions for a virtuous life....Publicly honoring the Ten Commandments is thus indistinguishable, insofar as discriminating against other religions is concerned, from publicly honoring God.
Here is the fatal flaw in Scalia's argument. He fails to include "another" monotheistic tradition within the Lowest Common Denominator, arguably the most important tradition for purposes of this discussion because it happens to be the personal religion of the first four presidents he mentioned: theistic rationalism. And the theistic rationalists did not necessarily believe that "the Ten Commandments were given by God to Moses." Nor did they ever say so in their public invocations of God.
These Founders did believe in a God, in fact believed that reason discovered God exists and grants men unalienable rights. But reason, not revelation is where ultimate truth is to be found. And these Founders disbelieved a great deal of revelation which they regarded as either unreasonable or unproven. And Moses divinely receiving those Commands was one of those truths about which our Founders were highly dubious.
For instance, here is Jefferson in an 1824 letter to Adams:
Where did we get the ten commandments? [The Bible] itself tells us they were written by the finger of God on tables of stone, which were destroyed by Moses; it specified those on the second set of tables in different form and substance, but still without saying how the other were recovered. But the whole history of these books is so defective and doubtful, that it seems vain to attempt minute inquiry into it; and such tricks have been played with their text, and with the other texts of other books relating to them, that we have a right from the cause to entertain much doubt what parts of them are genuine.
Adams likewise doubted that we had the right version of the Ten Commandments.
When and where originated our Ten Commandments? The Tables and The Ark were lost. Authentic copies, in few, if any hands; the ten Precepts could not be observed, and were little remembered.
If the Book of Deuteronomy was compiled, during of after the Babilonian Captivity, from Traditions, the Error or Amendment might come in there.
– John Adams to Thomas Jefferson, Nov. 14, 1813.
There is nothing in the private writings or public acknowledgments of the two other Presidents that contradict Jefferson's and Adams' sentiments here. They were all men of reason. Therefore, if we include the creed of the first four Presidents as part of the lowest common denominator of monotheism, we would have to exclude the notion that the Ten Commandments were divinely given by God.
What we would be left with in our LCD is this: There is a God; He grants us unalienable rights; He is concerned about human beings and will intervene, especially if we don't respect the unalienable rights of others and nothing more. The first four Presidents never more specifically defined God's attributes when publicly acknowledging Him.
In other words, a generic natural religion founds America's public order. "Nature" meaning what is knowable through reason, not revelation. Revealed religion is to be consigned to the private sphere of society (as Harvey Mansfield, Michael Zuckert and Walter Berns argue that our Founders, after Locke, intended).
"Natural religion," as it were is the religion of "all good men." And it does not teach that God revealed the Ten Commandments to Moses. Reason never "discovered" or "confirmed" that.