I want to explore this comment a little more as this has actually been suggested by a few others:
The idea is that doctrines which some derive from the Bible but are not actually in God's Words are theories while differences also exist based on interpretations of the Biblical text.
@37818 argued IF Atonement theories (specifically the Penal Substitution Theory of Atonement) are theories then so also is the idea that God is One, Jesus and the Father are One, and the Holy Spirit is the Spirit of God would also be a theory.
First off, I think it is important to consider exactly why Atonement theories are theories (they are so for different reasons).
Several Atonement theories (like the Moral Influence Theory, Ransom Theory/ Christis Victor) are theories be caused they prescribed a primary way of viewing the Atonement.
Scripture does describe the Atonement BUT the Bible does not offer one aspect over another.
Other Atonement theories (Satisfaction Theory, Penal Substitution Theory) are theories because they seek to explain the Atonement in a way not presented in the Biblical text.
Now....we come to the idea that God is One, Jesus and the Father are One, and the Holy Spirit is the Spirit of God.
Why is this not a theory?
The reason is each of those claims are stated in Scripture.
To clarify - nobody is looking for these things in one verse.
Nobody is saying the actual word "trinity" needs to be used.
Yes.
That God is One, the Son is God, the Spirit is God, the Father, Son, and Spirit are not the same (the Father sent the Son, not Himself...).
Now, if you are relying on philosophical ideas about "person" then maybe you do hold to a theory there as well.
But the basic doctrine of the Trinity is God is One, Jesus is God, the Holy Spirit is God, and they are unique to one another.
That said, you make a good point.
Many do hold theories (their understanding) over God's Word.
If the argument goes to what constitutes a divine nature, what constitutes "person", the details of the hypostatic union, etc. then the doctrine is venturing into theory.
To quote without permission, so not cited, "There is only one God and he reveals himself in three forms -- Father, Son, and Holy Spirit (Gen 1:26; Mt 28:19; 2Co 13:14)."
"Modes" and "forms" are identical.
The issue isn't what is present but what is omitted. This is actually why "distinct persons" (a simple meaning) was used.
Doctrines need to be as complete as possible without exceeding Scripture.
For example, many atheists hold a doctrine of Jesus - that Jesus died on a cross.
But as a "doctrine of Jesus" it is problematic.
The fact there are at least three versions of Trinity that is deemed by the holders of their interpretation, the points disagreement could be regarded as theory.
Calling those who differ unorthodox.
I do not think so. Philippians 2:6, . . . Who, being in the form of God, . . . Philippians 2:7, . . . took upon him the form of a servant, and was made in the likeness of men: . . .