Trinitarian orthodoxy can be formulated on the basis of 3 sets of NT statements, each consisting of one primary and several supporting statements:
1. 'yet for us there is one God, the Father,' (1 Cor 8:6a)
"This is eternal life, that they may know You, the only true God," (Jn 17:3a)
Jesus answered, "The foremost [commandment] is, 'Hear this O Israel: The Lord our God, the Lord is one,'" (Mk 12:29)
[Here Jesus quoted the Shema: 'Shema Yisrael, YHWH eloheinu, YHWH echad.' (Deut 6:4).]
2. "I and the Father are one." (Jn 10:30)
[If Jesus was speaking in Hebrew, He probably said "Ani veha'av echad", ending with echad as in the Shema.]
In the beginning was the Word, and the Word was with God, and the Word was God. (Jn 1:1)
whose are the fathers, and from whom is the Christ according to the flesh, Who is God over all, blessed forever. Amen. (Rom 9:5)
Who, existing in the form of God, did not consider to be equal with God something to be grasped, (Phil 2:6)
No one has ever seen God. The only begotten God, the One who Is in the bosom of the Father, He has made Him known. (Jn 1:18)
Plus 5 passages where the Son is referred as "ho Theos" with a qualification: Mt 1:23; Jn 20:28; Tit 2:13; 2 Pet 1:1; 1 Jn 5:20.
3. The Son and the Father are really distinct personal subjects.
[While the above is not an NT statement, it is an unavoidable straightforward conclusion from many NT statements, such as:]
"You are My Son, the beloved; in You I am well pleased." (Mk 1:11b)
"For the Father loves the Son and shows Him all things that He does." (Jn 5:20a)
"As the Father knows Me, I also know the Father; and I lay down My life for the sheep." (Jn 10:15)
"But that the world may know that I love the Father, and as the Father has commanded Me, thus I do." (Jn 14:31)
"Now, Father, glorify Me together with Yourself, with the glory which I had with You before the world was." (Jn 17:5)
Correspondingly, there are 3 possible ways to fall from trinitarian orthodoxy, each by denying one of the statements:
- Denial that there is one God = tritheism
- Denial that the Son and the Father are one = Arianism
- Denial that the Son and the Father are really distinct = modalism or Sabellianism
Of those ways to fall, the first to occur historically was modalism (Sabellius fl. ca. 215), but it was Arianism which presented the biggest threat to orthodoxy and prompted the Church to elaborate a precise formulation of trinitarian doctrine.
Since Arians denied that the Son and the Father were one, they had to interpret Jesus' words "I and the Father are one." (Jn 10:30) in the sense of only moral oneness and not ontological oneness. To counter that eisegesis, the orthodox sought for the right word to add to Jn 10:30 to convey unequivocally the sense of ontological oneness: "I and the Father are one"... what?
We arrive at the same question from four key statements by Jesus of his divinity that were not mentioned above: the 4 times in which He explicitely applied to Himself the divine Name in the first person revealed in Ex 3:14: "Ehyeh", "Ego Eimí", I Am": Jn 8:24,28,58 & 13:19. If each of the Father and the Son names Himself "Ego Eimí", "I Am", then Each is a distinct "I" but Both are the same... what?
The natural answer to that question was the term ousía, which derived precisely from the verb "eimí", "to be". So, the initial orthodox answer to the Arian challenge was: the Father and the Son are one ousía and two prósōpa, one being and two persons.
Now, in contemporary language the term "being" is ambiguous, and in principle could convey the sense of either ousía or hypostasis (the latter as synonym of divine Person, as was officially used since 382), though it sounds more like the former, with which it is etymologically related.