Seventh-Day Adventist Beliefs: The Godhead
There is one God: Father, Son, and Holy Spirit, a unity of three co-eternal Persons. God is immortal, all-powerful, all-knowing, above all, and ever present. He is infinite and beyond human comprehension, yet known through His self-revelation. He is forever worthy of worship, adoration, and service by the whole creation.
--Fundamental Beliefs, 2
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I think I have heard some people have trouble with the concept of a God-trinity. Is there one God, or three? I think that saying that there are three Persons that make up one God is sort of a way to underline their unity. In a polytheistic worldview, it's possible for different deities to disagree with each other, to be at cross-purposes, and be generally discordant. By conglomerating the Father, the Son, and the Holy Ghost into one "Godhead", we emphasize that this kind of discord doesn't happen between them.
I know that likely isn't right, but that's the sense I make of the idea.