I'm not sure what you mean.
I was asking what Stiggy meant by "He is one". I understand what trinitarians mean by that but not what Stiggy means by it. I've never discussed this subject with him and not every trinitarian has a full understanding of trinitarianism.
My point was the very confusion expressed in the question "meant by 'He is one'" answered your question as to why God didn't just teach the Trinity of God plainly on day one. Such a revelation requires time.
I agree that God kept some things secret until his incarnation and that there was a progression of revelation which was revealed in the last days by the Jesus and the apostles. I will continue to argue that the way God presented himself as one person to his chosen people, the Israelites and their fathers, was a true representation of himself, who he is. The only thing that changed upon his incarnation was that he began to exist as a man and at the same time remained as he always had been transcendent to his incarnation. I use the kenotic theory to explain it.
If "God kept some things secret until his incarnation and that there was a progression of revelation", then your overarching argument questioning why Scripture presents God as singular, allowing for the misunderstanding of unitarianism, is answered. The truth of the Trinity was something kept "secret until his incarnation" only to be revealed later upon "a progression of revelation".
Did God present "himself as one person to his chosen people"? Or did he present himself as a singular God, and you just assumed that only one person was that singular God? BTW, your explanation that things changed upon his incarnation makes sense until one carefully studies the OT. The use of "us" and "our" in Genesis 1:26, YHWH calling down fire from YHWH out of heaven in Genesis 19:24, a man, who was God, wresting with Jacob in Genesis 32:30, and the commander of the Armies of YHWH meeting with Joshua in Joshua 5:13-15 in the context of knowing "No one has ever seen God; the only God, who is at the Father’s side, he has made him known." John 1:18 clarifies that this personal distinction existed prior to the incarnation as well. Not to mention verses like John 17:5 which teaches that Jesus shared Glory with the Father before the world was: "And now, Father, glorify me in your own presence with the glory that I had with you before the world existed." You can't have "I had with you before the world existed." unless the I and the you designates two persons before the world was, while still recognizing that Jesus is God and that there is only one God.
I've read some of the ECFs. I've read Against Praxeas by Tertullian a few times. He wrote it ~ 210. He was the first to coin the term, the Trinity. If you have read it, you would see that his explanation of the trinity is different than that of today in how he explains the development of the Son, the Word, and the inequality of the three persons.
I've read a book on the trinity by Roger Olson and Christopher Hall and the sections concerning the development of the Trinity in Philosophical Foundations for a Christian Worldview by Moreland and Craig and discussed it for hours online. The doctrine developed slowly over time and became fully expressed at the council of Nicea in 325 AD.
You forgot about Theophilus of Antioch writing in the late 2nd century. He defines the Trinity as God, his Word (Logos) and his Wisdom (Sophia)[ in the context of a discussion of the first three days of creation, following the early Christian practice of identifying the Holy Spirit as the Wisdom of God. He died in 182ad. He coined the term in Greek. Are you really squabbling over 10 years with Tertullian who wrote the first defense of the doctrine of the Trinity in Latin when your still over 120 years earlier than your 300 year number? Say 200 years at least, at least then your trying to be historically honest. BTY, there is no justification for claiming the doctrine of the Trinity was "fully expressed at the council of Nicea in 325 AD." The Trinity proper wasn't even the primary topic under discussion at Nicea. They were primarily focused on Chirstology and denying the heresy of Arianism. All the arguments rejecting the various types of Oneness were hammered out over the previous 100 years. The difference at Nicea was that the bishops had political protection in 325 to meet in an "ecumenical" council. Prior to this, regional councils and written theological debate answered these concerns as evidenced by Tertullian. Also, that "He was the first to coin the term, the Trinity." in writing in Latin and that "his explanation of the trinity is different than that of today" is inconsequential to justifying when the doctrine of the Trinity was coined. It still existed in the writing of Ignatius of Antioch who refers to "all three persons around AD 110, exhorting obedience to "Christ, and to the Father, and to the Spirit"". It still existed "towards the end of the first century, where Clement of Rome rhetorically asks in his epistle as to why corruption exists among some in the Christian community; "Do we not have one God, and one Christ, and one gracious Spirit that has been poured out upon us, and one calling in Christ?" (1 Clement 46:6)." (cf Wikipedia's essay on the Trinity).
The apostles didn't teach the doctrine of the trinity, imo. The doctrine of the trinity would have been foreign to them as Jews. If they truly thought that God was a trinity, they would have openly taught that doctrine since it would have been a new revelation given to them by Jesus or the Holy Spirit. They would have shared it with the church. A truth like that which revealed the nature of God as a trinity would have been taught more clearly. It would have been a foundational doctrine. The apostles revealed secrets that were kept hidden by God from the foundation of the world. The doctrine of the trinity was not one of them.
This is a claim easily justified or refuted by going to Scripture itself to see what it teaches. I always wonder why people think general proclamations like this are meaningful when they could just make their case Biblically. I've already presented some Biblical justification for my perspective. Until you choose to go to Scripture to justify this claim, I choose to ignore it as simply an expression of your opinion.
This is a declaration of your opinion. Nothing more. Again, I've already presented some Biblical justification for my perspective. Until you choose to go to Scripture to justify this claim, I choose to ignore it as simply an expression of your opinion.