How does the Bible teach the Trinity - from Genesis to Revelation?
How has Church explored and exegeted this Biblical teaching?
The Bible spends far more time expounding the Three who are the one LORD God - rather than exploring HOW these Three are one.
There is very little in the Bible about the Unity - the oneness - of the LORD God.
Only when we get to the fifth book of the Bible is there any explicit statement at all about the unity of the Three - and then, by using the Hebrew word "echad" we are simply told that the LORD is a compound unity, but HOW this is so, is not explored there at all...
Other than...
Well, LOVE is the proper response to such a unified LORD, which some have seen as a clue as to the deep nature of the eternal divine unity.
Deuteronomy 6:4-5 - "Hear, O Israel: The Lord our God, the Lord is one (a unity). Love the Lord your God with all your heart and with all your soul and with all your strength."
For Biblical, orthodox Christians it is necessary to give true, full expression to all that the Bible says about the Father, Son and Spirit - and this means that the Three must be expressed in such a way that they are distinct from one another as separate "persons" - "agents" - but at the same time they have to be so bound together that they can be genuinely said to be ONE God.
Distinction and Union are both necessary.
Distinction has to be expressed - but in such a way that all Three are fully, equally and eternally "divine" (whatever the word "divine" means). They cannot be SO distinct that they are no longer "homoousion" - sharing the same nature.
Union has to be expressed - but in such a way that all Three are eternally distinct such that we preserve the way that they speak to one another and that each can use the word "I" in a meaningful way.... so that personal love happens between the Three. Union cannot become so strong that the Three merge into one another.
We must leave aside the clear heresies of modalism (the ever-present temptation of Western Christianity) and polytheism (which includes Arianism, Jehovah's Witnesses and Mormonism).
Once we have turned away from these theological cesspits, there seem to be three main strategies to account for HOW the Three are One.
1. The word "God" refers to the single (even "simple") divine essence - and because the Three participate in this divine essence, the Three are one God. In other words, the shared divine essence or substance is why there are not Three Gods.
The unity of God is the divine essence.
2. The word "God" refers to the oneness of the society, communion, or fellowship (koinonia) of the Three. The Three share such a close and eternal fellowship, gaining their identity from the love that binds them together as one, so there are not Three Gods.
The unity of God is the divine society/communion.
3. The word "God" refers to the Father (the source of the Son & Spirit) - who ALWAYS and ETERNALLY acts through His eternally begotten Son in the power of the eternally proceeding Spirit - such that the one action of the one God is necessarily from the Father, through the Son and by the Spirit.
The unity of God is the Father - expressed in unity of operation - three Persons, one energy/work.
(Irenaeus - the two hands of the Father)
St Ambrose in "De Fide" 3:25 states "unity of Godhead is signified by unity in operation".
(The same conclusion is found in Gregory of Nyssa in his letter to Eustathius. We cannot know anything at all about the divine essence, so we cannot discover the unity of Trinity in a divine essence. However, because we see the Three always working together as one, we can conclude that they are one, sharing the same nature).
We know what something is by what is does: so pay attention to the actions of the Three to see how they are one - all doing the same one work of God.
Interestingly, there is an important difference between the Greek "ousia" and the Latin "substantia".
Ousia is a verbal noun - describing an ACTION (the action of being). This was preferred to a more abstract concept like "to on" (that which is).
A regular noun refers to a "thing", a "what" - but a verbal noun is tied to action. At the Council of Nicaea there was concern about using language that was too far removed from the simple activity language of the Bible.
Note: Those who find the unity of God in the divine essence tend to be LEAST comfortable with the distinctions between the Three Persons... and therefore open to occasional modalist accusations?
Those who find the unity of God in the divine fellowship may be MOST comfortable with the distinctions... and therefore open to occasional polytheist accusations?