The first quotes in this post (in blue text) are taken directly from the booklet, Is God A Trinity, printed in 2011, by United Church of God,
In the early church it wasn't long before true servants of God became a marginalized and scattered minority among those calling themselves Christians.
The doctrine of the Trinity is considered so sacred and fundamental that many churches ... view it as a litmus test for defining who is and who isn't a true Christian. For example, author and theology professor James White writes; "We hang a person's very salvation upon the acceptance of the doctrine ... No one dares question the Trinity for fear of being branded a 'heretic' ... We must know, understand, and love the Trinity to be fully and completely Christian".
Another author writes: "You cannot be saved if you don't believe in the Trinity".
The booklet continues with many more examples from teachers who insist that the unscriptural belief in the Trinity is essential to being a Christian. Ironically, Christian churches say they base their doctrines on what the Bible teaches.
The following quotes (in blue) are from the United Church of God web page: http://www.ucg.org/booklet/god-trinity/trinity-biblical/
The International Standard Bible Encyclopedia acknowledges that "'trinity' is a second-century term found nowhere in the Bible, and the Scriptures present no finished trinitarian statement" (1988, Vol. 4, "Trinity," p. 914). It further states that "church fathers crystallized the doctrine in succeeding centuries"—long after the apostles had passed from the scene.
The HarperCollins Encyclopedia of Catholicism states: "Today, however, scholars generally agree that there is no doctrine of the Trinity as such in either the OT [ Old Testament ] or the NT [ New Testament ] . . . It would go far beyond the intention and thought-forms of the OT to suppose that a late-fourth-century or thirteenth-century Christian doctrine can be found there . . . Likewise, the NT does not contain an explicit doctrine of the Trinity" (Richard McBrien, general editor, 1995, "God," pp. 564-565).
The New Encyclopaedia Britannica, in its article on the Trinity, explains:"Neither the word Trinity nor the explicit doctrine appears in the New Testament . . . The doctrine developed gradually over several centuries and through many controversies . . . It was not until the 4th century that the distinctness of the three and their unity were brought together in a single orthodox doctrine of one essence and three persons" (1985 edition, Micropaedia, Vol. 11, p. 928).
The New International Dictionary of New Testament Theology points out that "primitive Christianity did not have an explicit doctrine of the Trinity such as was subsequently elaborated in the creeds of the early church" (Colin Brown, editor, Vol. 2, 1976, "God," p. 84).
Martin Luther, the German priest who initiated the Protestant Reformation, conceded, "It is indeed true that the name 'Trinity' is nowhere to be found in the Holy Scriptures, but has been conceived and invented by man" (reproduced in The Sermons of Martin Luther, John Lenker, editor, Vol. 3, 1988, p. 406).
Professor Charles Ryrie, in his respected work Basic Theology, writes: "Many doctrines are accepted by evangelicals as being clearly taught in the Scripture for which there are no proof texts. The doctrine of the Trinity furnishes the best example of this. It is fair to say that the Bible does not clearly teach the doctrine of the Trinity . . . In fact, there is not even one proof text, if by proof text we mean a verse or passage that 'clearly' states that there is one God who exists in three persons" (1999, p. 89).
Ryrie goes on to state: "The above illustrations prove the fallacy of concluding that if something is not proof texted in the Bible we cannot clearly teach the results . . . If that were so, I could never teach the doctrine of the Trinity" (p. 90).
Professor Erickson further states that the Trinity teaching "is not present in biblical thought, but arose when biblical thought was pressed into this foreign mold [of Greek concepts]. Thus, the doctrine of the Trinity goes beyond and even distorts what the Bible says about God"
This post could be made almost infinitely long by quoting Bible teachers who say they believe that the Trinity exists, but who, at the same time admit that such a teaching is not logical, not believable or taught in the Bible.