If there is a belief that is central to Israel’s identity in the Old Testament, it is this: God is one (Deut. 6:4). In contrast to the nations surrounding Israel, nations that worshiped many gods, Israel was set apart as a people who worshiped only one God. They were to be monotheists.
But we should add that true monotheism is not merely the belief that there is one God. It means also that this God is one. Theologians call this God’s simplicity. This doesn’t mean that God lacks depth. Rather, simplicity refers to God’s oneness. He is not a God made up of parts, let alone divided by parts. It’s not as if you could add up all God’s attributes to get the sum total we call “God.” Instead, God is one. His attributes are His essence1 and His essence His attributes. All that is in God simply is God. To say, then, that God is one not only means that there is only one true God but also that this God is one in essence.