Galatians 3:19-20 (ESV)
Why then the law? It was added because of transgressions, until the offspring should come to whom the promise had been made, and it was put in place through angels by an intermediary.  Now an intermediary implies more than one, but God is one.
But God is one. Monotheism is at the heart of Christianity. There is only one true God, the one who created the world. It is revealed throughout the Old Testament and more explicitly that there is more to this one God than perhaps is seen at first blush. He is not a monolithic God. That is he shouldn’t be compared to the simplicity of a stone. Perhaps we don’t know how God is three in one, but we do know that he is three in one. His word reveals this to us. We recognize the Father, and the Son, and the Holy Spirit as being one. This oneness goes beyond just being on the same page with each other, like the one will of a foot ball team to win. Paul emphasizes this oneness. God is one. There is one God. They are one in substance, separate in persons. There is no way of reading this emphasis that lends itself to saying “one in will and purpose,” as the Arians before and the Mormons today. Paul is even combating that idea when he talks of an intermediary implying more than one. It does imply more than one, it implies at a minimum two. And it would have been easy for Paul, if indeed it were true, to say there are more than one, but they are united in will and purpose. However Paul does not say that at all. Rather he takes this opportunity to emphasize that God is one, there is but one God. He does not say the gods are one, but God is one. And there he leaves it. It is enough. God is one, as the Shema, Deuteronomy 6:4, the ancient creed of Israel, whom we now are, also emphasizes. The religion of Israel has not changed. God is one.