Ehrman is right about the late origin of trinitarian dogma, but due to his stance about the Christ’s resurrection and the fallibility of scripture, he won’t get far with believers.
I believe that the concept of the trinitarian dogma came from Athanasius
Before he was 20 years old, he wrote two treatises: “Against Pagans” and “The Incarnation” in which he argued that Jesus is God: “con-substantial and co-eternal with the Father ... as is the whole, so also is the part.” The Catholic Encyclopedia states that his two treatises “were admittedly written about 318 before Arianism as a movement had begun to be felt.”
In these two papers, Athanasius promoted Jesus past all the competition. He maintained that Jesus was not merely a man (because of his miracles); nor a prophet no matter how highly inspired. He was not a man who became a god, as Roman emperors were deified by vote of the Senate; nor a god who became a man, as Mithras was said to have done; nor a demigod like Hercules, the hybrid offspring of a god and a human mother; nor one of many gods, like Zeus; nor even the only-begotten, subordinate Son of the only God. None of these was sufficient. Athanasius said that Jesus was the “Artificer” who created all things in heaven and earth, the “re-creator” of human beings, and that while he was operating his human body, he was also operating and sustaining the rest of the universe as the Supreme Being. [Athanasius; Christian Classics Ethereal Library]
Athanasius apparently started with his conclusion, “Jesus is God,” and backed in all the arguments he could muster in support of that conclusion. The use of this rhetorical method is called sophistry. He justified his thesis in terms of its effect on believers: “Apart from belief in the Incarnation, we [they] cannot ultimately believe in Christ as Redeemer.”
And he used this in his battles against all other forms of Christian belief then existant until they were all declared as “Heresys” in 380 by Emperor Theodosius.