Basically, Eusebius of Caesarea, c. 300, took a poll of bishops. He had sifted through about 100 documents that, in some way or other, had ever been used or cited as Christian literature by any purportedly Christian authority. In terms of selection of texts, he cast his net fairly wide. In terms of who he polled, he was a bit more narrow. He polled acknowledged orthodox bishops, but not representatives of any other Christian group, of which there were a good many.
He divided the texts up into four categories: genuine, disputed, spurious, and rejected. In order to make it into the "genuine" category, which meant that it was actually written by an apostle or one of his close assistants, it had to have been acknowledged as genuine by every bishop who ever lived. If even one bishop, living at any time, did not recognize a certain book as "genuine," then it went into the "disputed" category. (His information wasn't perfect--he didn't have access to every possible document--but he did an exceptionally thorough job, considering his limitations.)
The one exception is the book of Revelation, which Eusebius said was disputed by some, but he nonetheless included it as "genuine" for a reason known only to him. (Eusebius sometimes has to be taken with a certain "elasticity.")
The book of Revelation was strongly opposed by Dionysius, the influential bishop of Alexandria, who argued that it was not written by John, son of Zebedee, one of Jesus' disciples. Moreover, he said, it could not be interpreted literally. If it was, then it was in agreement with the heretic Cerinthus, who argued that Christ would return, establish a literal kingdom on earth, and reign for 1000 years.
The "disputed" category included James, Jude, Second Peter, and and Second and Third John--books which are in our present-day New Testament. Into the "spurious" category were books like Acts of Paul and the Revelation of Peter and many others, most of them not because of theology, interestingly enough, but because of the lack of witness to their actual apostolic authorship.
In the "rejected" category were the Gospel of Thomas, an early and apparently influential writing used by Christian gnostics, and the Gospel of Peter, which was rejected because it received only one "no" vote, that of Serapion, the bishop of Antioch.
Constantine became Emperor of Rome in AD 313. One of the first things he did was to legalize Christianity, and grant religious freedom in the Empire. (We rightly celebrate religious freedom in the United States, but must acknowledge that the Emperor Constantine did it long before we did. Unfortunately, it didn't last.)
With a friendly Emperor now in power, the Christians faced a radically changed situation. The persecutions had ended less than a decade before Constantine gained power. Now, seemingly all of a sudden, they had gone from persecuted to favored--from underdog to top dog.
Constantine favored the orthodox Christians in many ways. He built a magnificent church in Constantinople, and other churches in various cities of the Empire. He gave the bishop of Rome a palace to live in. He gave direct state support to Christian clergy--(which, not surprisingly, increased the number of people who considered themselves Christian clergy). He intervened in theological disputes, including the Arian controversy in the middle east, and what would become the donatist controversy in Africa.
Constantine's motive was unity. He believed that the "Supreme God"--it took awhile before Constantine used explicitly Christian language--would favor the Empire if there was unity in the church. He was more concerned with harmony than theological exactitude. Constantine appears to have been quite sincere in his belief that God would bless him and the Empire if there was concord in the church catholic.
One of Constantine's many benefactions was to commission the production of 50 Bibles. He would employ the best in the business to do it. As Constantine himself put it, they would be produced "by professional transcribers thoroughly practiced in their art."
Constantine sent the order to Eusebius of Caeserea, who, not surprisingly, constructed these 50 Bibles to include the books he, on the basis of his research and theological perspective, had concluded were the books that ought to be considered sacred scripture. These included all the books he had identified as "genuine," plus the books of Revelation, Jude, James, 2 Peter, and 2 and 3 John.
By ordering 50 Bibles with the imprimatur of the Emperor, Constantine, in effect, "standardized" the New Testament. The 27 books chosen by Eusebius became what we today call "the canon," which is our present day New Testament.
Photo: Der Spiegel, a page of the New Testament found at St. Catherine's monastery in Egypt, which may date from the time of Constantine