You come to explained “mystery” that in the NT describes something that was not understood before, but is now being revealed and you further elaborate that is some truth that had been hidden in the past but was eventually revealed by God. In the Old Testament the prophets known God is only alone while in the NT scripture tells us God always specified as the Father. How’s that?
Now for clarification, since both Trinity and Incarnation are a mystery. I explained Incarnation as a divine mystery or mystery of godliness as we can read in scripture. Further, it is mysterious how can God always specify in the NT as the Father will manifest in the flesh(Christ) while you explain Trinity as a mystery as you present this explanation “one person with one being (body), and I can't conceive of a being who is three persons with one being (Spirit) like God.”
By the way, is this also part of the mystery of Trinity that we might not understand exactly when you tell this to me…”God does not have a body; he is a spirit. Therefore, there is only one spirit."... Jesus was more than that. His body is a real human body just like any other man”? I want to clarify it from you.
I am not sure I fully understand this dialogue.
My point was that the Triune God was not fully revealed in OT Scripture but he is revealed in NT Scripture. Jesus was "God with us". The Holy Spirit is another advocate like Jesus , but not Jesus.
So, the distinctions were not clear until the Incarnation and the Day of Pentecost (Acts 2).
The NT does in fact use the word "God" in many contexts to indicate the Father, but it also displays the Son as being God (John 1:1-3).
These notes may answer your question.
1 Timothy 3:16 3:16 mystery. See note on v. 9. Here Paul refers to things formerly hidden but now made visible by God’s revelatory activity, described in a luminous six-line composite portrait. This activity centers on (1) Christ’s incarnation; (2) Christ’s resurrection, which confirmed his identity as the Son of God (Rom 1:4); (3) the angelic recognition at Christ’s resurrection (Matt 28:2) and ascension (Acts 1:10); (4) the expansive proclamation as Christ’s followers preach throughout the world; (5) Christ’s transformative presence in far-flung believing communities as the gospel message goes forth; and (6) Christ’s glorious triumph, dramatized in the ascension (see Luke 24:50–53; Acts 1:9–11 and notes). There is not, then, strict chronological order to the sequence. Many see poetic or hymnic features in the verse. godliness. Its essence is Christ’s work, received by faith (see note on 2:2).
(NIV Biblical Theology Study Bible)
John 1:1-5 In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was in the beginning with God. 3 All things were made through him, and without him was not any thing made that was made. 4 In him was life, and the life was the light of men. 5 The light shines in the darkness, and the darkness has not overcome it.
(ESV)
John 1:14 14 And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth.
(ESV)
This verse says that the Word (Jesus) was with (in a face to face relationship) God (the Father), and the Word was God (in terms of deity).
The Greek word for "with" (pros) indicates a face to face relationship.
Regarding the word "God" I am speaking specifically of the Triune God. The Triune God does not have a body; he is Spirit.
Jesus is both God (meaning he is one Person of the Triune God) and he is glorified man. Jesus is a union of God and man. He is fully God and fully man.
A Person with two natures is not easy to explain. And, Jesus is such a person.
He is called the Theanthropos - the God-man.
By the way, these topics are something that are the topics of church councils, and the Creeds speak of them. Heretics attempt to capitalize on issues like this in order to disillusion the person concerning Christianity, and to drag them into their cultic groups.
You might want to study the Creeds in this regard, and research their Scriptural support.
I think there's some level of difficulty posed by language barriers. If you want a better explanation, I can likely refer you to a Filipino pastor who can explain these concepts to you.