“Hear of Israel, Yahweh our God, Yahweh is One.” (Deuteronomy 6:4)
“You believe that God is one? You do well; the demons believe that also…and they tremble.” (James 2:19)
“A mediator means there is more than one, but God is one.” (Galatians 3:20)
“And now, Yahweh our God, please save us from his hand, that all the kingdoms of the earth will know that you, Yahweh are God and you alone.” (2 Kings 19:19)
“One who sacrifices to a god shall be destroyed; Yahweh alone — to Him alone!” (Exodus 22:20)
The Bible is clear that while God exists in three persons, he is one and exists over and above all things both that he has created and that has been created in the imaginations of men. The question sometimes gets asked, “Aren’t there other gods that people worship?”
To answer that question properly, one must first offer a definition of what constitutes a “god.” If, by a “god,” what you mean is anything that people worship (rightly or wrongly), then I suppose that we could say that there are many gods. There would be the obvious things that come to mind: images of Buddha or of the Catholic saints (or Mary), Asherah or Totem poles, and idols to Hindu gods like Shiva. Money falls into this category as people will pursue it with all of their energy rather than pursuing God with all of their energy. Famous people are another illustration of the gods people bow down before and fawn over.
Self also can be defined as a god by this standard. We live in a culture of narcissism and people obsess over their looks, their image, their pleasure, and their self-gratification. Body art, plastic surgery, and even “gender re-assignment” have become commonplace and drug use has become an epidemic. On one extreme, people are working to genetically modify embryos to emphasize “more desirable” traits and on the other hand, children are being murdered in the name of “Family Planning.” Worse yet, like the Israelites in Canaan who ignored and participated in the paganism in their midst rather than pushing it out of the land, the Church largely ignores these false gods in our midst rather than pushing these false gods out of our churches.
And further, when you broaden the definition of a word so much that it can mean almost anything, then the word ultimately means nothing. To put it another way, our ability to communicate with one another is predicated on the idea that words have a limited semantic range. If “god” is defined as anything that man bows down to, everything becomes a god and the word is ultimately meaningless.
To this end, let me offer a more narrow definition from a Christian perspective. And this is to borrow the definition that St. Anselm used when he was devising his “Ontological Proof” for the existence of God. His definition is: “God is a being which no greater being can be imagined.” To narrow that down even more, one might point out that this definition demands that God not be one of a subset of gods but instead, that God is in a class of his own — the being par excellence. By this definition, there can only be one God — and this is the definition found in the Bible. Whether people worship themselves, the works of their hands, or demons, none fo these are in the same class as God — he is truly unique and alone in terms of his person and character; none is like him, no not one.
And to this end, both Christian theology and Hidelberg Catechism, question 25 insist on God being defined as One being even when we are defining him as three persons. It is an essential of the faith and non-negotiable in Christian theology.