All beings have a spirit. Some beings are spirit; some beings are corporeal. All beings but one are created.
Human beings are corporeal, and naturally inhabit the physical realm. The beings described in scripture as personed but incorporeal are called elohim, and naturally inhabit the spiritual realm.
God is an elohim; or God is spirit. God is the Elohim; He alone natively holds all authority.
Uniquely among all beings, God is a single being of three persons. All other single beings are a single person. Single person/single beings are called individuals, because they are an indivisible, singular person. God is not an individual, but a triune being.
Similarly, God is the only uncreated being, and is uniquely infinite rather than finite.
The second person of the singular, triune being, God, inhabited a corporeal form at a specific point in history, and is now, again uniquely, both fully God and fully human. We call that person of the God-being, Jesus. He remains one being with the Father and the Spirit.
We capture all of this meaning (and more) when making the concise statement, “God is holy.” In other words, God is wholly other, completely righteous, entirely separate, and uniquely unique.
So now you know; God is Trinity: the only entirely righteous, uncreated, eternally existent, tri-personed, singular Being.
For a defense of this explanation see: the Bible.
For additional amplification see: