After the discussion about why the bible uses 'let US make them in OUR image' in the book of Genesis, I went and got myself a copy of 'The Other Bible'...it's a collection of Gnostic and early Kabbalahic texts, as well as the dead sea scrolls and some gospels that were ultimately rejected by Christianity.
I started to read The Secret Book Of John, and part way into the first couple of chapters I got another idea as to why that particular phrasing was used
Yahweh, the God that Christianity and Judaism both view as being the one and the only God, the one who created the earth and man....was himself a creation. The offspring of Sophia, mother of wisdom and her consort, the Father of Light (although legend has it that the Father of Light was unaware of Sophia's creation). He is describes himself as a 'jealous God' because he knows that he's not the sole divine power.
Gnostics believe that sin and evil came about not because of Adam and Eve's disobedience, but because of God's very act of creating the world and man...which he did with arrogance, ignorance and vanity. God took light particles from his mother, Sophia, and trapped them inside his human creations...causing them to want to return to the Father of Light. Adam and Eve began the process of redemption with their first act of disobediance to the Creator God by eating from the Tree of Gnosis (knowledge)...also known as The Tree of The Thought of Light.
So, it would seem that some of the ancients believed that God, Yahweh, was not alone. Which might explain why he refers to himself in the plural.
I also found the following in the 'Odes of Solomon':
'A cup of milk I was offered
And I drank its sweetness as the delight of the Lord.
The Son is the cup
and he who was milked is the Father
and he who milked him is the Holy Ghost
The Holy Ghost opened the Father's raimewnt
and mingled the milk from the Father's two breasts
and gave that mingling to the world, which was unknowing.
Those who drink it are near his right hand.'
(Ode 19, The Odes of Solomon)
Hmm...a God that breastfeeds his creations.....that's something to think about too.....