The New Testament was written within the period of half a century, several hundred years after the completion of the Old Testament. The manuscripts were completed within about 60 years of Jesus' crucifixion. Despite being younger, the New Testament canon has less certification. Jesus' use and endorsement of the Hebrew Scriptures as the authoritative Word of God certifies the Old Testament, though He did anticipate the New Testament in His promise of the Holy Spirit who teaches all things. Before original manuscripts were found, we had approximately 15,000 complete manuscripts or partial quotations of the New Testament today that were written or copied within the first several hundred years following Christ's death--thousands more than any other writings from that era.
The litmus test for canonicity is the recognition at the very time of writing that the apostles were the authentic spokesmen of God. We can be assured that the books were received by the church of the apostolic age precisely when they had been certified by an apostle as being thus inspired. The apparent variation, relative to geographic area, in acknowledgment of some of the epistles may reflect the localized nature of this attestation. The geographic range of the New Testament was far broader than the Old Testament, after all. Conversely, that all 27 books of the now universally received New Testament were ultimately agreed upon is evidence that proper attestation was indeed confirmed after rigorous investigation.
The canon was formally recognized in the 4th century.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the New Testament by Milton C. Fisher. Pages 65-66; FaithQuest - Student Edition (DP) by Sharon Berry and Ollie Gibbs (2008). Page 7.)
Timeline
First Century
The earliest books were early Paul epistles and James (A.D. 48-60). The Gospels were A.D. 60-100, as well as the other NT books. 1 Thessalonians is possibly the first book written.
For the first century, the Gospels and rest of the New Testament were written then had a local and independent existence in the constituencies for which they were composed before they began circulating throughout the church. Their biblical authority was established within the books themselves.
Paul's letters were preserved by the communities or individuals to whom they were sent, but by the end of the 1st century, his surviving correspondence was being collected into a Pauline corpus and quickly circulated - first as 10 then as 13 (added the 3 Pastoral Epistles). They were ordered by length. (and today, our NT epistles are ordered by length, except for Galatians before Ephesians.)
The early church inherited the Hebrew Bible or the Greek Septuagint and gained the Gospel collection, the Pauline corpus, and Acts to serve as a link between the two, and soon set these new evangelical and apostolic writings alongside the Law and the Prophets. Justin Martyr wrote that the memoirs of the apostles and writings of the prophets were read in their Sunday meetings. (Apology 1.67)
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. Biblical Languages by Larry Walker. Section One: The Authority and Inspiration of the Bible. Page 9-10; The Canon of the New Testament by Milton C. Fisher, Pages 68-69.)
The early church had closer ties and greater information than we have today, so they examined the testimony of the ancients to discern which were authentic and authoritative books by their apostolic origin. We accept it today as historical fact, though the Reformation reassessed the books through intuition and spiritual discernment. The Church was convicted that the apostles were uniquely authorized to speak in the name of the One who possessed all authority. Jesus personally authenticated his words to the believers, and Jesus authorized his apostles to speak in his name. Apostolic speaking on behalf of Christ was recognized as the Word of God in the church, whether in personal utterance or in written form. Incontrovertible consistency of doctrine in all the books was perhaps a subordinate test, but historically, the procedure was one of acceptance and approval of those books that were vouched for by knowledgeable church leaders. Second and third generation Christians looked back to apostolic words as the very words of Christ, not exactly establishing the canon but simply bearing witness to the authority of the word of Christ.
Mark's association with Peter and Luke's with Paul's gave them apostolic approval. Epistles like Hebrews and Jude were tied in with the apostolic message and ministry.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the New Testament by Milton C. Fisher, Page 76-77.)
Early Second Century
By the beginning of the 2nd century, they were more widely known and recognized as authoritative. The John Rylands papyrus indicates John was revered and copied by A.D. 125, within 30-40 years of the apostle's death. There is also evidence all the Gospels and Pauline letters were known and used throughout the church within 30 years of John's death. The authority of some of the smaller letters was questioned in some quarters for another 50 years, but this was only due to the uncertainty of their authorship in those locales.
Eventually, the Gospels were brought together and circulated as a fourfold record, detaching Acts from Luke and circulating on its own.
Acceptance of the New Testament Canon
The first three outstanding church fathers, Clement, Polycarp, and Ignatius, used the bulk of the New Testament in a revealingly casual manner, indicating authenticated Scriptures were accepted as authoritative without argument. In their writings, only Mark (which closely parallels Matthew's material), 2 John, 3 John, Jude, and 2 Peter were not clearly attested. Epistles of Ignatius (A.D. 115) have correspondences in several places with the Gospels and seem to incorporate language from the Pauline letters. The Didache, perhaps earlier, makes references to a written Gospel.
Distinction between Scripture and Church Writings is recognized
Importantly, Clement, Barnabas, and Ignatius all draw a clear distinction between their own and the inspired, authoritative apostolic writings. In Epistle of Barnabas 4:14 (A.D. 130), we first find the formula "it is written" used in reference to a New Testament book (Matthew 22:14 in this case). Before this, Polycarp used a combined Old Testament + New Testament quotation and refers to them as "these Scriptures" in Epistle to the Philippians 12:4. Papias, Bishop of Hierapolis, uses Matthew and Mark as the basis of exposition in circa A.D. 130-140, indicating his acceptance of them as canonical. The Gnostic Gospel of Truth (A.D. 140) treats its use of canonical New Testament sources as authoritative in such a comprehensive way that it warrants the conclusion that there was a New Testament compilation corresponding closely with our own in Rome at this time. Citations are made from the Gospels, Acts, the letters of Paul, Hebrews, and Revelation.
Marcion defined a limited canon of his own in 140, hastening the need for orthodox believers to declare the canon.
When Justin Martyr described worship services in the mid second century, he put apostolic writings on par with those of the Old Testament prophets, since the same voice spoke through them. He also was free in his use of "it is written" with quotations from New Testament Scriptures.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. Biblical Languages by Larry Walker. Section One: The Authority and Inspiration of the Bible by F. F. Bruce. Page 9-10; The Canon of the New Testament by Milton C. Fisher, Pages 68-72.)
The heretical movement of Montanus was an impetus toward the recognition of a closed canon of the written Word of God. He taught that the prophetic gift was never withdrawn, so he could add to the Word of God. The pressure to deal with him intensified the search for a basic authority, and apostolic authorship or approval became recognized as the only sure standard for identifying God's revelation. Even within Scripture, 1st century prophets were subordinate and subject to apostolic authority (1 Corinthians 14:29-30; Ephesians 4:11).
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the New Testament by Milton C. Fisher, Pages 75.)
Late Second Century
By the end of the second century, the New Testament was recognized alongside the Old Testament as Scripture. As Christianity spread, there were Old Latin and Syriac versions of the New Testament by A.D. 200, and a Coptic one within the following century. The Syriac omitted 2 Peter. Also biblical commentaries were written during this time.
Irenaeus quoted from almost all the New Testament on the basis of its authority and inspiration. He also is the earliest surviving witness to recognize all four Gospels. Tatian in Diatessaron attested to the equal status the four Gospels had in the church by A.D. 170.
The Muratorian Canon dates from A.D. 170 and includes Matthew and Mark (inferred to be in destroyed sections), Luke, John, Acts, 13 Pauline letters, Jude, 1 John, 2 John, and Revelation. It states, "We accept only the Apocalypse of John and Peter, although some of us do not want it to be read in the Church." 2 Peter may be the Peter's Apocalypse? The list also rejects by name various heretical leaders and their writings.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. Biblical Languages by Larry Walker. Section One: The Authority and Inspiration of the Bible by F. F. Bruce. Page 9-10; The Canon of the New Testament by Milton C. Fisher, Pages 68-69; 72-73.)
Third Century
During the third century, the books were collected into a complete "New Testament," and other Christian literature was excluded from the canon.
The outstanding name of the third century is Origen who made critical studies of the New Testament text alongside his work on the Hexapla and wrote commentaries and homilies on most of the books of the New Testament, emphasizing their inspiration by God.
Origen's pupil Dionysius of Alexandria indicates that while the Western church accepted the book of Revelation from the first, its position in the East was variable. In the case of the letter to the Hebrews, the situation was reversed. It proved to be more insecure in the West than in the East. When it comes to other contested books, among the so-called "Catholic Epistles" Dionysius supports James and 2 John and 3 John but not 2 Peter or Jude. In other words, even at the end of the third century, there was the came lack of finality about the canon as at its beginning.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the New Testament by Milton C. Fisher, Pages 68-69; 73.)
Fourth Century
Early in the 4th century, the canon settled. During the fourth century, the church fathers stated that conclusions regarding the canon have been reached that indicate acceptance by the whole church. The canon became fixed.
Eusebius, bishop of Caesarea, sets forth his estimate of the canon in the early part of the 4th century in his Church History (3, ch. iii-xxv).
Universally agreed upon as canonical were the four Gospels, Acts, the letters of Pauls (including Hebrews with question about his authorship), 1 Peter, 1 John, and Revelation. The majority accepted 2 Peter, 2 John, 3 John, and Jude. The Acts of Paul, the Didache, and Shepherd of Hermas were classified as "spurious," and other writings were listed as "heretical and absurd."
In the latter half of the 4th century, the New Testament canon finds full and final declaration. In his Festal Letter for Easter, 367, Bishop Athanasius of Alexandria set to eliminate once and for all the use of certain apocryphal books. "Let no one add to these; let nothing be taken away" is the earliest extant document that specifies our 27 books without qualification. At the close of the century the Third Council of Carthage (A.D. 397) decreed that "aside from the canonical Scriptures nothing is to be read in church under the Name of Divine Scriptures." This too lists the 27 books of the New Testament.
Christianity suddenly advanced under Emperor Constantine and the Edict of Milan, A.D. 313, allowing the New Testament books to be spread to the East. He assigned Eusebius to prepare 50 copies of the Divine Scriptures, effectively establishing a standard that legitimized all formerly disputed books. In the West, Jerome and Augustine were the leaders in legitimizing the canon. Publication of the 27 books in the Vulgate version virtually settled the matter.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the New Testament by Milton C. Fisher, Pages 68-69; 74-75.)
Criticism
Some feel there is an inadequacy to the solutions the Reformers proposed to their own questions. Was Luther's test for canonicity valid? Was Calvin's assertion that Christians can use spiritual discernment to validate books reliable?
Some fear the Church Fathers operated on wrong information. It's possible several of the New Testament books were not written until after the apostolic age, and some have questionable authorship.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the New Testament by Milton C. Fisher, Pages 77-78.)