Is there a simple definition for the Trinity?
God is one being who exists eternally as three distinct persons — the Father, the Son, and the Holy Spirit. That is the whole definition: one “what” (being) and three “whos” (persons). Not one God wearing three costumes, and not three parts of God, but one God in three Persons — held together because Scripture holds them together.
Yes. Here it is: God is one being who exists eternally as three distinct persons — the Father, the Son, and the Holy Spirit.
That is the whole definition.
Most of the confusion about the Trinity comes from skipping the definition and reaching for a picture instead. Two pictures get used more than any others. The first is water: water can be ice, liquid, or steam — one substance in three forms. The second is an egg: shell, white, and yolk — one egg in three parts. Both are offered with good intentions. Both teach the reader something false.
Take the water. The same water is not ice and steam at the same time; it changes out of one form and into another. Apply that to God and you get one God taking turns — Father for a while, then Son, then Spirit. That mistake has a name: modalism. It leaves you with three costumes worn by one person instead of three persons. Scripture rules it out at the river Jordan, where all three are present at the same moment and each is doing something different.
Matthew 3:16–17 (NKJV)
When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. And suddenly a voice came from heaven, saying, “This is My beloved Son, in whom I am well pleased.”
The Son stands in the water. The Spirit comes down upon Him. The Father speaks from heaven. Nobody is taking turns, and nobody is talking to Himself.
Now the egg. A shell is not an egg. A yolk is not an egg. Each is a piece, and you need all the pieces before you have the whole. Apply that to God and each person becomes a third of God, with none of them fully God on His own. That mistake has a name too: partialism. Scripture rules this one out as well. Writing about the Son, Paul says:
Colossians 2:9 (NKJV)
For in Him dwells all the fullness of the Godhead bodily.
All the fullness — not a portion of it. And what is true of the Son is true of the Father and of the Holy Spirit.
The lesson is not that we need a better picture. It is that we should stop hunting for one. Nothing God has made is a working model of God, because God is not one more item in the world alongside the things He made.
Isaiah 40:18 (NKJV)
To whom then will you liken God? Or what likeness will you compare to Him?
So instead of a picture, hold two plain statements from Scripture together. The first is that there is only one God, and the Bible says it flatly:
Deuteronomy 6:4 (NKJV)
Hear, O Israel: The LORD our God, the LORD is one!
The second is that the same Bible calls the Father God, calls the Son God, and calls the Holy Spirit God — while keeping them distinct from one another. The Son prays to the Father (John 17:1). The Father sends the Spirit in the Son's name (John 14:26). There is one God, and He is Father, Son, and Holy Spirit.
Two words — being and person — make this manageable. Being answers the question what something is. Person answers the question who someone is. You are one being and one person — one what, one who. God is one being and three persons — one what, three whos.
The three persons are distinct, but they never work apart from one another. You can see that in how a sinner is saved. Paul walks through it in the opening of one letter:
Ephesians 1:4, 7, 13 (NKJV)
just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love … In Him we have redemption through His blood, the forgiveness of sins, according to the riches of His grace … In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise
The Father chose. The Son redeemed. The Spirit sealed. Three persons, three works, one salvation — and not one of them acting alone. The Father chose you in the Son. The Son obeyed the Father. The Spirit seals what the Father purposed and the Son accomplished.
One question is left. If God is one being and three persons, do we now understand God? No. What we have is a definition, not an explanation. Scripture states plainly that God is one, and it states just as plainly that the Father, the Son, and the Holy Spirit are each God and each distinct from one another. It never tells us how. That gap is not a contradiction. A contradiction would say two things that cancel each other out — one God and three Gods, or one person and three persons. The doctrine of the Trinity says neither of those. It says one being, three persons: two different words describing two different things, held together because the Bible holds them together. So we are not asked to explain how God can be this. We are asked to believe what He has revealed about Himself.