The Law of Moses made a "doctrinal statement" about Israel's "one God" and immediately said how they were to respond to this "doctrine" - by loving God. Jesus emphasised that the way to do this was through following Him and loving others. The doctrine has no relevance without the behaviour.So what was the "doctrinal statement" being made in the sh'ma - "'Hear, O Israel, the Lord our God, the Lord is one" - and how does this connect to the behaviour which is commanded?
I suggested that the sh'ma could be simply a statement of monotheism - i.e. there is one God - or it could be saying that Israel's God is unique, a one-of-a-kind God. These statements are not mutually exclusive of course, so the sh'ma could be saying both. However, as someone else pointed out so well, the first half of the sh'ma has no purpose without the second and Jesus gave this doctrine a greater depth of meaning by adding a new dimension. The sh'ma connected the Oneness of God with the command to love Him with our whole being, and Jesus added to this the commandment to love our neighbour as ourselves.
It seems to me then that the Jesus Creed is saying something like this:
When we put the "doctrinal statement" that "God is One" in its context we see that it is really a statement about the unity which should exist in creation, and between the created and its Creator. It is not so much a statement about God as it is about God's relationship with His creation.
God is One. On one level He is also "at one" with His whole creation because in Him we live and move and have our being*. There is a connectedness between everything in creation, and between the created and the Creator, and the created only finds true meaning when it realises and acknowledges this connection and responds by loving the Creator with its whole being. When the connection - the "union" or Oneness - is lost on a spiritual level between the Creator and the created, it can be restored again, but only through love. The Creator loves His creation, and when the created loves its Creator the connection begins to be restored on the spiritual level. The connection is made complete when the created discovers their part in Creation and loves themselves, others, and God. Everything is made complete, perfect and whole through love**.
God is One - He is whole, complete, and perfect. His purpose in creation is for everything to be part of the Oneness, to be "at one" with Himself in perfect unity, or Oneness, through love.
* Acts 17:28
** Aim for perfection, listen to my appeal, be of one mind, live in peace. And the God of love and peace will be with you. (2 Corinthians 13:11)
And over all these virtues put on love, which binds them all together in perfect unity. (Colossians 3:14)
There is no fear in love. But perfect love drives out fear, because fear has to do with punishment. The one who fears is not made perfect in love. (1 John 4:18)