"Hear, O Israel: The LORD our God, the LORD is one." Deuteronomy 6:4
A. There is but one only, the living and true God.
God has both naturally and supernaturally revealed that there is only one God.
Agnostics will give credit to "a higher power" (note in the singular) and atheists, though by definition deny the existence of a deity, if posed with the question, would say that if there was a God, they would refer to him as being one. It is assumed, by the light of nature, whether religious, agnostic or atheist, there is a god, and He is the only God (Isaiah 44:6).
But what of the religious people? Christians are not the only group of people who believe in one god. What of Judaism and Islam? They all claim the belief in the God of Abraham. Are we all in agreement that we believe in the same God? Some would agree, but the Divines did not, because the Bible does not agree. The Bible speaks of the one true living God (Jeremiah 10:10). This means there are gods who are dead, meaning they are not real, they are idols (Jeremiah 10:8).
Who is the one true living God? Yes, He is the God of Abraham, but He is even more. He is the Father of our Lord Jesus Christ (Matthew 3:16-17). He is the one who gave His only Son to save us from our sins (John 3:16).
"Hear O Israel, the Lord our God, He is one," is not a passage of mathematics, but one of love. Not only is He the one and true God, He is our God. He is one in our mind, heart and soul, the utmost of our affections. Hear, O people of God, the Lord our God, He is one.