As I've mentioned before, the doctrine of Trinity is a powerful and totally non-dualistic image of reality. The Father, Son, and Spirit, as we named them, share in an endless mutuality of giving and receiving of an infinite love. We too are allowed to participate in this oneing of diversity through unitive consciousness. Reality is radically relational, and the power is in the relationships themselves. If reality is created on the model of the Trinity where Yahweh even speaks in the plural (Genesis 1:26), then intercommunion is the first and final shape of the universe.
The Celts readily welcomed the Christian Trinity perhaps because their own deities took shape in threes. For example "the goddess Bridget appeared in three forms: the goddesses of fire, of poetry, and of fertility, all three named Bridget" . Celtic prayers and poetry are full of references to each member of the Trinity, as in this psalm by Columba (late 6th or early 7th century):
The High First-Sower, the Ancient of Days and unbegotten,
was without any source, limit, or foundation in the beginning and is,
and will be throughout unending ages forever;
With him is the only-begotten one, the Christ;
And the co-eternal Holy Spirit in the constant glory of the Godhead.
We do not claim that there are three gods; rather we declare that God is one,
But not at the expense of believing in three most glorious Persons. 
I believe the flowing waterwheel of the Trinity was intuited in the circle and "medicine wheel" of many Native American religions. Many Native images and metaphors take the shape of a circle, an endless ring symbolizing the interconnectedness of all things. In the words of Black Elk, an Oglala holy man:
You have noticed that everything an Indian does is in a circle, and that is because the Power of the World always works in circles, and everything tries to be round. In the old days when we were a strong and happy people, all our power came to us from the sacred hoop of the nation, and so long as the hoop was unbroken, the people flourished. The flowering tree was the living center of the hoop, and the circle of the four quarters nourished it. The east gave peace and light, the south gave warmth, the west gave rain, and the north with its cold and mighty wind gave strength and endurance. This knowledge came to us from the outer world with our religion.
Everything the Power of the World does is done in a circle. The sky is round, and I have heard that the earth is round like a ball, and so are all the stars. The wind, in its greatest power, whirls. Birds make their nests in circles, for theirs is the same religion as ours. The sun comes forth and goes down again in a circle. The moon does the same, and both are round. Even the seasons form a great circle in their changing, and always come back again to where they were. The life of a man is a circle from childhood to childhood, and so it is in everything where power moves. Our tepees were round like the nests of birds, and these were always set in a circle, the nation's hoop, a nest of many nests, where the Great Spirit meant for us to hatch our children. But the Wasichus [white men] have put us in these square boxes. Our power is gone and we are dying, for the power is not in us any more. You can look at our boys and see how it is with us. When we were living by the power of the circle in the way we should, boys were men at twelve or thirteen years of ago. But now it takes them very much longer to mature. 
When we forget the roundness of life, the inter-being of all creatures and the Creator, we lose our sense of true identity and belonging--to that very circle. Tomorrow I'll share the gift of initiation as a way of bringing us back to home and center.