For a reminder of what a mass noun is, wikipedia has a summary: it is a noun which grammatically does not get counted, like “sand” or “meat” or “furniture”.

The First Commandment being “You shall have no other gods”, it would seem silly to try to argue anything other than that “God” is a count noun and that true believers will believe that there is only one. But that would be making things too simple.

Luther writes in his Small Catechism that this commandment should be interpreted primarily as a commandment place nothing in our lives as a higher importance than God. So this leaves room for interpreting God not as a single countable entity, but instead as a non-countable entity. Indeed, The other things that we might be tempted to place above God — wealth, fame, knowledge, etc. are themselves mass nouns, so Luther’s interpretation of the first commandment actually demands that we view God in its capacity as a mass noun to make a proper comparison.

Taking this approach to God is instructive when trying to rationalize the Holy Trinity. Christians have long been ridiculed when trying to explain how one God can really be three Gods. Even the best of explanations are little more than enthusiastic handwaving: More indicative of the folly of the creed than anything. But if God is a mass noun, then the Trinity is its enumerator: just as one can count sand by enumerating the grains (grains of sand) , or count corn by enumerating kernels or ears or bushels, Christian mythology counts God as Father, Son, and Holy Ghost.

It also adds a new dimension to the institution of Ceremonial Deism. When I see “In God We Trust” written on a dollar bill, I don’t interpret that as being an endorsement of monotheism, because “God” isn’t singular or plural — it is a mass noun.