“Indeed, even though there may be so-called gods in heaven or on earth—as in fact there are many gods and many lords—yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist” (1 Corinthians 8:5-6).
The short definition is monotheism is the belief that there is only one God. However, there are many beings called god or elohim in the Old testament. Some of these are Angels, some are demons, and even humankind is called gods.
In order to better understand monotheism, we must first define theism. Theism is the belief in an Infinite and Eternal God who is both transcendent and imminent.
How clearly demons, angels and humans do not meet this definition of theism. Therefore, there is only one God who meets this definition. At least according to the Old and New Testament. And that is the God of Israel and the Christian Church.
Therefore, monotheism, properly defined, is a biblical and Christian doctrine. Christianity has always been considered a monotheistic religion, as soon as the word monotheism came into use. To deny this is to deny the facts.