Shorter Catechism Q5

Are there more Gods that one?
There is but One only, the living and true God.

Recorded in Deuteronomy 6:4, Moses said to the people of Israel, "Hear, O Israel: The LORD, our God, the LORD is one." This is the most basic of beliefs about the nature of who God is, and it is a foundational truth of all three of the religions that stemmed from Abrahamic roots, Judaism, Islam and Christianity. This is a complex doctrine, although it seems simple, because as a Christian, I know that God has one substance but also three persons. Many of the names for God that are given to Him in the Old Testament relate to his nature, such as "Lahai roi - The Living One who sees me" (see Genesis 16:13-14). He is also described by his relationships to people: "the God of Abraham, Isaac and Jacob", (Exodus 3:15), "the God of Israel" (cf 2 Chronicles 11:16), "the God of our fathers" (cf 2 Chronicles 15:12). His eternal name Yahweh or Jehovah reveals his self existent nature: "I AM WHO I AM", as He told Moses.
It is vitally important that I worship the One true living God. It's a waste of time to worship a god made by human hands or constructed in human minds, because "what are not gods" (2 Chronicles 13:9) are nothing but figments. The One true living God has revealed Himself supremely through the Bible.