"Ye know that ye...carried away unto these dumb idols, even as ye were led" (1 Cor. 12.2).

The word monotheism is derived from the Greek μόνος meaning "single" and θεός meaning "God". If God is gods then God is not single in the true sense of the meaning.

The concept sees a gradual development out of notions of henotheism (worshiping a single god while accepting the existence or possible existence of other deities) and monolatrism (the recognition of the existence of many gods, but with the consistent worship of only one deity). Substance monotheism, found in some indigenous African religions, holds that the many gods are different forms of a single underlying substance.

Satan's working hard. But what does God have to say? First off, Jews today who reject Christ still hold to One Being in Monotheism, so for you to come along and say they never held to One uncreated Being, seems dishonest and disingenuous. Judaism is one of the oldest known monotheistic faiths. The best-known Jewish statements of monotheism occur in the Shema prayer, the Ten Commandments and Maimonides' 13 Principles of faith, Second Principle:
God, the Cause of all, is one. This does not mean one as in one of a pair, nor one like a species (which encompasses many individuals), nor one as in an object that is made up of many elements, nor as a single simple object that is infinitely divisible. Rather, God is a unity unlike any other possible unity. This is referred to in the Torah (Deuteronomy 6:4): "Hear Israel, the LORD is our God, the LORD is one."
"Well, we all know that an idol is not really a god and that there is only one God and no other" (1 Cor. 8.4). There is no other! Men are called gods because they have the knowledge of good and evil like God. Don't read into it more than that. There are many claims, as well, of being gods, but they are just claims.

Jesus said, "I and the Father are one" being (John 10.30), "one Lord" (Deut.6.4) "from everlasting to everlasting" (Ps. 90.2). "Believe, that the Father is in me, and I in him" (John 10.38) proves that the Father and the Son are One Being. God would never have said this if the Father and Son were of different substance and separate beings.