St Ambrose lived in the 4th century from 340-397. One of his most brilliant works is an "Exposition of the Christian Faith".
To grasp his opening chapter, it is very important to understand the energy/essence distinction in ancient Christian theology - a distinction that is also in the Bible.
We cannot know the "essence" of God.
We cannot find out "what" God is, as if we could analyse God as we analyse a sample in a laboratory.
No! We can never DIRECTLY access the essence of the Father, Son and Spirit.
BUT, we do encounter this Living God in action.
The Bible presents the Three in action - and that is how God is revealed: in energies (workings) not essence.
The Bible does not even speak about the "divine essence", but instead shows us the Three Persons working together at all times.
We can share in the life of God as we are caught up into these energies of God - the activity of the Father through the Son by the Spirit.
God in action is how we encounter God: we do not encounter or understand an abstract "essence".
"God in action" is always the Father working through the Son in the power of the Spirit.
There are NOT three different "workings" of God.
There are not three separate "energies".
The Father works through the Son by the Spirit - and that is the ONE working of the ONE God.
God is ONE - and so the Father ALWAYS works through the eternal Son by the eternal Spirit.
ONE work.
ONE energy.
We see this in Genesis 1:1-3.
Elohim (plural noun) created (singular verb).
THREE 'workers' but ONE work.
ONE energy.
So, the heretics often spoke as if the Son had a separate work to the Father - as if they did different things.
Because they divided the "work" of God into different "works" (plural), they ended up with three gods!
It is vital to show that all the way through the Bible, from Genesis to Revelation, there is only ever ONE divine energy - ONE singluar work of God - and it is always from the Father through the Son by the Spirit.
Listen carefully to the way Ambrose explains the Trinity against heretics and pagans.
This is from chapter 1 of his great work.
"6. This is our declaration of faith: we believe that God is One, not separating His Son from Him as the heathen do, nor denying, like the Jews, that the Son was begotten of the Father before all ages and later born of the Virgin.
We do not, like Sabellius, confuse the Father with the Word, claiming they are the same Person.
Nor do we say the Son began in the Virgin’s womb - like Photinus says.
Nor do we, with Arius, imagine multiple divine energies and thus more than one God, as the heathen believe.
For it is written: “Hear, O Israel: the Lord your God is one Lord.”
God and Lord are titles of majesty and power, as He Himself says: “The Lord is My name” (Exodus 3:15) and as the prophet declares: “The Lord Almighty is His name.”
He is God and Lord because He rules over all,
sees all things, and is feared by all alike."
[Ambrose is showing that the names "God" and "LORD" are not about the "essence" of God but titles describing the action of God.]
[Note also that Ambrose says that God "sees all things" - because he seemed to believe that the Greek word "Theos" is connected to the word for "seeing".]
"If God is One, then the name and energy of the Trinity are one.
As Christ said: “Go, baptize the nations in the name of the Father, and of the Son, and of the Holy Spirit” (Matthew 28:19)—in the name, not names.
[One single name - not three names]
Christ also said: “I and the Father are One” (John 10:30), meaning no separation in energy or nature.
Yet, Jesus speaks of more than one - “WE are" - so that the distinction of Father and Son is recognized.
[One work - one energy - but more than one agent/person]
The perfect Father begot the perfect Son (Matthew 5:48).
They are One not by blending Persons, but by unity of nature.
Thus we confess one God, not two or three, rejecting the Arian heresy that divides the Godhead into separate Gods.
The Lord’s command to baptize in the name of the Father, Son, and Holy Spirit affirms this truth."
The painting is by Claude Vignon in 1623.
On the table is a knotted whip to show how Ambrose fought against heresy. Notice the three knots in the whip - to show how his weapon was the doctrine of the Trinity!