polytheism
The belief that there are many gods; the worship of multiple deities. 

• Scripture teaching against polytheism:

“You are my witnesses,” declares the LORD,

“and my servant whom I have chosen,

that you may know and believe me

and understand that I am he.

Before me no god was formed,

nor shall there be any after me. (Isaiah 43:10 ESV)

For all the gods of the peoples are worthless idols,
but the Lord made the heavens. (Psalm 96:5 ESV)

Therefore, as to the eating of food offered to idols, we know that “an idol has no real existence,” and that “there is no God but one.” For although there may be so-called gods in heaven or on earth—as indeed 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 8:4-6 ESV)

• From the Second Helvetic Confession, Chapter 3:

Truly we detest many gods because it is expressly written: “The Lord your God is one Lord”(Deut.6:4). “I am the Lord your God. You shall have no other gods before me” (Ex. 20:2-3). “I am the Lord, and there is no other god besides me. Am I not the Lord, and there is no other God beside me? A righteous God and a Savior; there is none besides me” (Isa. 45:5, 21). “The Lord, the Lord, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness” (Ex. 34:6). 

• From God Is Love by Gerald Bray:

When speaking about pagans to the Romans, Paul described them in these words:

What can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, the did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things. [Romans 1:19-23]

The picture is hardly flattering, but [this passage] tell[s] us something of great importance about so-called “primitive” or pagan [or polytheistic] religious practices. The pagans knew God in their hearts, and God continued to speak to them by the providential care that he lavished on them. They went wrong because they had no access to spiritual knowledge and found themselves looking for substitutes within the material universe.