From the divine inflow into human souls, discussed just above, it follows that in everyone there is an inner voice saying that God exists and that there is one God. Nevertheless there are people who deny God, people who worship nature as God, people who worship many gods, and people who worship idols as gods. The reason for this is that they have let worldly and bodily perspectives block off the inner reaches of their reason or intellect and obliterate their first childhood idea of God; they have rejected religion from their heart and have put it behind them.
The Christian confessional creed shows that Christians acknowledge one God and shows how they view the unity of God:
The catholic faith is this, that we venerate one God in a trinity, and the Trinity in unity. There are three divine persons, the Father, the Son, and the Holy Spirit, and yet there are not three gods; there is one God. The Father is a person, the Son another, and the Holy Spirit another, and yet they have one divinity, equal glory, and coeternal majesty. The Father, then, is God, the Son is God, and the Holy Spirit is God; but just as Christian truth compels us to confess each person individually as God and Lord, so the catholic religion forbids us to say that there are three gods or three lords.
This is the Christian faith regarding the unity of God. (In the chapter that discusses the divine Trinity [Sections 163–184] you will see that the trinity of God and the unity of God presented in this confession are incompatible.)
The other nations in the world that possess religion and sound reason agree that there is one God: all Muslims in their countries; the Africans in the many countries on their continent; and the Asians as well, in the many countries on theirs. So too do modern-day Jews. In the Golden Age, the most ancient people who were religious worshiped one God, whom they named Jehovah. So did the ancient people in the following age, up to the time when monarchies were created. In the time of the monarchies, upper levels of the intellect that had previously been open, and had been like sanctuaries and temples of worship to the one God, were increasingly closed off by worldly loves, and then by bodily loves. The Lord God, in order to unblock those upper levels of the intellect and restore worship of one God, instituted a church among the descendants of Jacob and set the following precept above all the other precepts in their religion: “There is to be no other God before my face” (Exodus 20:3).
“Jehovah,” which he named himself anew for the Jews, means the highest and only Being, and the origin of everything that exists and occurs in the universe. People in the preclassical period acknowledged Jove as the highest God (perhaps so named from Jehovah), and deified many others who made up his court. In the age that followed, however, sages like Plato and Aristotle admitted that the other [Olympians] were not gods but different properties, qualities, and attributes of the one God, called gods because there was divinity in each of them.
from True Christianity, Volume 1, Section 9