Some ill-informed and ill-intentioned folks speak folly about the formation of the New Testament Canon. Here is a primer for the ignorant.

(1) Key Dates:

First Century: All NT books were completed by the end of the first century.

Second to Third Century: Muratorian Canon (~AD190) listed 23 of the current 27 NT books, except 3John, Hebrew, and 1&2Peter. NT books used by early church father Origen (185-254) had most of them too, except Hebrews, James, 2Peter, 2&3John, Jude.

Third to Fourth century: NT used by early church father Eusebius (265-339) included all except James, 2Peter, 2&3John and Jude.

Fourth century: NT canon was first listed by early church father Athanasius in AD367, and subsequently affirmed for the West by the both the Council of Hippo (AD393) and of Carthage (AD397). Those few books that were in dispute were finally included in the canon.

(2) Key Reasons:

Marcion (c.110-160), a heretic, prompted the early church to think about forming a New Testament. He rejected all OT books, and NT books that have Jewish audience in mind such as Matthew, Mark, Acts, and Hebrews, and several other books such as pastoral letters that he disagreed.

Another troublemaker, Montanus, forced the church to think about closing the canon. He came as a self-named prophet to the stage in the middle of second century (156-172), with his two prophetesses, Prisca and Maximilla.

As one can see, Constantine had little to do the NT canon formation. The canon formation started at least 100 years before his time (272-337), endured waves after waves of persecutions by various Roman emperors before him, and finished ~60 years after his death.