God is one in being and 3 in person.
A being is that quality or that essence what makes you what you are, a person is that quality, or that essence that makes you, whom you are.
For example, what I am is a human being, and who I am is Abraxos.
The being = I am a human being
The person = I am Abraxos
We are human beings and we share that amongst us, but none of you share essentially the type of person that I am. We are all different persons. So the characteristic of a being is very different of a person. I am one being with one person, God is one being with three persons - Father, Son, and Spirit. Is there anything like that on this earth? No. But with God all things are possible. Not only that but there is a revelation in the trinity - we see the deeper truths of God's nature, of what does it mean to be the father?, what does it mean to be the son?, what does it mean to be the Spirit? These are different roles in the trinity and often times people see the terms Son and impute inferiority to the Son. In a sense this is true and there are scriptures that show this, "For my father is greater than I."
For example, my father is the CEO of his company and I am under him, he's at the top. In that sense he is greater than I am, but he is a human being just as I am and we are of the same blood. So my father's being is essentially equal to mine. He's a human being and I am a human being, we are equal in that sense; but his role is greater than mine, and I am inferior in that sense.
So when Jesus says "the father is greater than I", the being is equal but the role is different. And this is how the trinity all comes into focus when we read the Bible.
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