All of the sources you read from below are Trinitarians. The thing that is striking to me is that they are being honest about the origins of the Trinity of three persons in one essence. You cannot simply go to the Bible and extract this theological position. The prophets, Jesus and the apostles were all believers that God is one single person. As the church tried to explain the relationship between Jesus Christ and God they began to draw from philosophy to arrive at a proper explanation. However, they failed to stay with the simple truth of Scripture that God is one, he is a Spirit that is omnipresent, omnipotent and omniscient. Jesus is the Messiah, the Son of God, the express image of the invisible God. The fulness of the invisible God was manifested through the Son of God.
It is not heresy to question doctrines that are not contained directly in Scripture. The Father, Son and Holy Spirit are not three separate persons that exist eternally side by side next to each other in fellowship. This is tri-theism not the biblical one true God. Father, Son and Spirit are three different manifestations of the one God. The Father is the source, the invisible one, the creator and the sustainer of all things. The Son is the visible expression and manifestation of the Father. The Son is the visible of the Father. All that the Father is is displayed through the Son. The Holy Spirit is what God is, God is a Spirit (John 4:24) and is the very presence, nature and power of God working in his people. Father, Son and Spirit are not three persons, but three manifestations of the one true and living God. Of course there is much more detail to the doctrine than this, but this is the most basic premise. Read for yourself what the various sources state about the Trinity.
James Hastings: “It has been customary to trace the institution of the practice to the Words of Christ in Matthew 28:19, but the authenticity of this passage has been challenged on historical as well as textural grounds. It must be acknowledged that the formula of the threefold name, which is here enjoined, does not appear to have been used by the primitive church, which so far as our information goes, baptized ‘in’ or ‘into’ the Name of Jesus, or Jesus Christ, or the Lord Jesus, without any reference to the Father or the Spirit” (Dictionary of the Bible, p. 88).
Scribners: “The original form of words were into the Name of Jesus Christ or Lord Jesus. Baptism into Trinity was a later development” (Dictionary of the Bible, Vol. I, p. 241).
Canney Encyclopaedia: “The early church always baptized in the Name of the Lord Jesus until the development of the Trinity; afterward they were baptized in the Name of the Father, Son, and Holy Ghost” (p. 53).
American Encyclopaedia, International Edition: “The term Trinity was used by Theophilus of Antioch in AD 180? (Vol. 27, p. 116).
Encyclopaedia Britannica: “The triune and Trinity formula was not uniformly used from the beginning, and up until the third century, baptism in the Name of Christ only was so widespread that Pope Stephen, in opposition to St. Cyprian, said that baptism in the Name of Christ was valid. But Catholic missionaries, by omitting one or more persons of the Trinity when they were baptized, were anathematized by the Roman church. Now the formula of Rome is, “I baptize thee in the name of the Father, and in the name of the Son and in the name of the Holy Ghost” (11th Ed., Vol. 3, p. 365-366).
Encyclopedia of Religions: “Persons were baptized at first in the Name of Jesus Christ, or ‘in the Name of the Lord Jesus.’ Afterwards, with the development of the doctrine of the Trinity, they were baptized in the Name of the Father, and of the Son and of the Holy Ghost” (p. 53).
New International Encyclopaedia: “The Trinity doctrine. The Catholic faith is this: ‘We worship one in Trinity, but there is one person of the Father, another of the Son, and another of the Holy Ghost. The glory equal—the majesty co-eternal.’ The doctrine is not found in its fully developed form in the Scriptures. Modern theology does not seek to find it in the Old Testament. At the time of the Reformation, the Protestant Church took over the doctrine of the Trinity without serious examination” (Vol. 22, p. 476).
Hastings Encyclopedia of Religion: “Christian baptism was administered by using the words ‘in the Name of Jesus.’ The use of a Trinity formula of any sort was not suggested in the early Church history. Baptism was always in the Name of the Lord Jesus until the time of Justin Martyr when the Triune formula was used” (Vol. 2, p. 377-378, 389. )
“NAME was an ancient synonym for “Person.” Payment was always made in the name of some person referring to ownership. Therefore one being baptized in Jesus’ Name became His personal property. “Ye are Christ’s.” (Acts 1:15; Revelation 3:4; I Corinthians 3:23).
LIFE Magazine: “The Catholics made this statement concerning their doctrine of the Trinity to defend the dogma of the assumption of Mary in an article by Graham Green: ‘Our opponents sometimes claim that no belief should be held dogmatically which is not explicitly stated in the Scripture but the Protestant churches have themselves accepted such dogma as the Trinity for which there exists no such authority in the Gospels’” (October 30, 1950, Vol. 29, Number 18, p. 51).
Catholic Encyclopaedia: “The true doctrine of the sacrament of baptism is not taught by the Roman church. Baptism given by heretics in the name of the Father and of the Son and of the Holy Ghost with the intention of performing what the church performs, is not true baptism” (Vol. 2, p. 259).
New Catholic Encyclopedia: “With regard to the form used for Baptism in the early church, there is the difficulty that although Matthew (28:19) speaks of the Trinitarian formula, which is now used, the Acts of the Apostles (2:38; 8:16; 10:48; 19:5) and Paul (I Corinthians 1:13; 6:11; Galatians 3:27; Romans 6:3) speak only of Baptism ‘in the Name of Jesus.’ Baptism in titles cannot be found in the first centuries. . .” (McGraw Hill Publishing, p. 59).
William Phillips Hall: “In this very ancient version (Syriac Peschito Version) which is believed by good authorities (Gwilliam, Boners, and others) to represent a text much older that of the Greek manuscript from which our English Old Testament was largely derived, ‘The Name of the Lord Jesus Messiah or Christ’ appears in all four readings given (Acts 2:38; 8:16; 10:48; 19:5)” (A Remarkable Discovery, p. 70).
International Encyclopaedia: “The doctrine of the Trinity did not form part of the Apostles’ preachings, as this is reported in the New Testament” (First Edition, Vol. 18, p. 226).
New International Standard Bible Encyclopedia: “The term ‘Trinity’ was originated by . . . Tertulian, a Roman Catholic church father. No record of the Trinitarian formula can be discovered in the Acts of the Apostles. . . At the time of the Reformation, the Protestant Church took over the doctrine of the Trinity without serious examination” (Vol. 1, p. 396).
“Because the Trinity is such an important part of later Christian doctrine, it is striking that the term does not appear in the New Testament. Likewise, the developed concept of three coequal partners in the Godhead found in later creedal formulations cannot be clearly detected within the confines of the canon.” “Trinity,” in The Oxford Companion to the Bible, Oxford University Press, 1993, p. 782.
“The adoption of a non-biblical phrase at Nicea constituted a landmark in the growth of dogma; the Trinity is true, since the Church — the universal Church speaking by its Bishops — says so, though the Bible does not! We have a formula, but what does that formula contain? No child of the Church dare seek to answer.” “Dogma, Dogmatic Theology,” in Encyclopedia