The books of the New Testament were written shortly after Christ's resurrection. Most scholars agree that all of the books included in the New Testament cannon were written by 80 A.D.

The determination of which books to include in the New Testament was a refining process that took place mostly over the first 300 years of the Church. During that time, early church leaders determined which writings were the authoritative testimony of the gospel of Jesus. They did this by looking at how orthodox the teachings were, how widely accepted the writings were, and how close were the authors to the apostles.

The books to include in the New Testament have been widely disputed throughout the first 1500 years of Christianity. Many rejected what we call accepted books for fear of the influences of Marcion and the Gnostics. Other books, such as the Shepherd of Hermas and the First Epistle of Clement were part of some lists of New Testament books for several centuries, depending on the location of the church. Martin Luther determined certain books were less authoritative than others if they contradicted the message of salvation by grace alone.

So the New Testament as we know it is technically from about 350 AD, but it had an elasticity for several centuries after that.