Question: I thought this might interest you: the new catholic encyclopedia states: the formulation "one God in three persons" was not solidly established, certainly not fully assimilated into Christian life and its profession of faith, prior to the end of the 4th century. But it is precisely this formulation that has first claim to the title "the Trinitarian dogma". Among the apostolic fathers, there had been nothing even remotely approaching such a mentality or perspective. (1967) vol 14, p299.
The new encyclopedia Britannica says: Neither the word Trinity, nor the explicit doctrine as such, appears in the New Testament, nor did Jesus and his followers intend to contradict the shema in the Old Testament: "hear, o Israel: the Lord our God is one Lord" (Deut 6:4). The doctrine developed gradually over several centuries and through many controversies. By the end of the 4th century the doctrine of the trinity took substantially the form it has maintained ever since. (1976) vol 10 p126.
Philippians 2:5-6 is plain enough: "Keep this mental attitude in you that was also in Christ Jesus, who, although he was existing in gods form, gave no consideration to a seizure, namely, that he should be equal to God.
I could go on, but I think that will do for now, feel free to ask if you want more, I will leave the last words to Paul "However, the inspired utterance says definitely that in later periods of time some will fall away from the faith, paying attention to misleading inspired utterances and teachings of demons, by the hypocrisy of men who speak lies, marked in their conscience as with a branding iron;" 1 Timothy 4:1,2.
Response : With all due respect to the reference sources you quote, this is the standard response of those who see Christianity as a religion developed historically by men rather than as the truth of the one true God in the Person of His Son Jesus Christ. The Trinity has existed since before the universe was created. Without the Trinity, there would be no universe. Records of the early Church are few and far between - apart from the Bible. The fact that there exists very little contemporary documentary evidence of the beliefs of the early Church does not mean anything beyond that. If you read the letters of Polycarp (a disciple of John), you will see that he expresses himself in a way indistinguishable from the phraseology of the New Testament.
So we are back to two things: 1) the divine reality, and 2) the Bible which accurately reflects the divine reality. You cannot see the Trinity, the Father, Son and Spirit, but they exist - that is the divine reality. You only exist because of them. We Christians walk by faith, what we believe, not by sight, what we can see (2Cor.5:7; cf. 2Cor.4:18), so that we know, understand and believe the truth of the Trinity regardless of what any human authority may say. As to the Bible, it definitely reflects and clearly teaches the divine reality of the Trinity. The Bible, written primarily in Hebrew and Greek, does not use this Latin derived word, but even the most casual student of the biblical text is well aware that Jesus Christ, that the Father, that the Holy Spirit are all ubiquitously present in its pages. The Bible clearly teaches that they are and that they are God. The fact that the luctations of the later church-visible to understand these truths and to express them in a formulaic (and often imperfect) way are not present within the verses of scripture is merely a further indication of the inspired nature of the Bible (and of the inability of any human effort to approximate it). Search the scripture and you will see that from the very beginning the truth of the Trinity is present for all to see (cf. Gen.1:26: "in our image"; or Is.63:8-12, where the Angel of His Presence, His glorious Arm, and His Spirit are co-authors of Israel's deliverance). In the interest of time and space I cannot reproduce here what you need to consider on this important subject without shortchanging it. Please see the links:
Explaining and Defending the Trinity and the Person of Christ.
The Trinity in Scripture.
Bible Basics: Theology, section II: the Persons of God: the Trinity.
The Trinity in Isaiah 63:10-15.
The One True God and the Trinity in the Old Testament.
Finally, it is clear to me from your closing quotation that your agenda in assailing the truth of God in three Persons is to question the divinity of our Lord and Savior Jesus Christ. Jesus is a true human being, but He is also truly and fully God. See the links:
Where does the Bible teach that Jesus is God?
How can Jesus be a man and God at the same time?
The Divinity of Jesus Christ
Jesus is God and man.
Jesus is God.
Since the earliest days of the Church, denying the divine nature of Jesus has been a prominent and common heresy. You cannot truly follow Him without accepting the truth of Him. To be a believer in and a follower of Jesus you have to believe who He is (truly human and truly divine) and what He has done (dying for your sins and rising from the dead for your eternal life) - and you have to follow Him faithfully in this life.
Here's a better translation of that Philippians passage you give:
You too should have this attitude which Christ Jesus had. Since He already existed in the very form of God, equality with God was [certainly] not something He thought He had to grasp for. Yet in spite of this [co-equal divinity He already possessed], He deprived Himself of His status and took on the form of a slave, [and was] born in the likeness of men. He humbled Himself, becoming obedient to the point of death, even [His] death on [the] cross [for us all].
That God should take on true humanity and constrain Himself to human limitations, suffering, striving, fighting and dying for us - that we might live with Him forever, is truly "good news". Only God could do this, and scripture is quite clear to all those who are seeking the truth that Jesus was, is, and always will be God. He also became, of His own free will, a true man, having taken on this mission and accomplished the sacrifice of the cross to cleanse us from our sins and so to give us the opportunity to come to God through Him and His work.
If you are truly looking for answers, read your Bible. The Trinity (along with many truths of scripture) may not be found in secular works, but scripture has all the answers for all who seek Him in truth.
Yours in Him who is the way, the truth and the life, our Lord Jesus Christ.