The apostle Peter testifies to the inspiration of Paul’s letters and calls them Scripture. This is a huge statement for a Jewish man to equate a contemporary’s writings to be equal to the Old Testament.

Our beloved brother Paul, according to the wisdom given to him, has written to you, as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures. (2 Peter 3:15-16)

The Apostle John attributes the same blessing upon his letter as Old Testament prescribes upon its readers.

Blessed is he who reads and those who hear the words of this prophecy, and keep those things which are written in it; for the time is near. (Revelation 1:3)

But what about all the books of the New Testament not written by the apostles? Is there any way to test their validity? Yes, it is the test of the apostles’ endorsement of these books. All other books of the New Testament was written by associates or disciples of the apostles. Mark, who wrote the gospel bearing his name, was closely associated to Peter and later Paul. Therefore, his main source for his gospel was a live witness of the life of Christ. Some have actually called the book of Mark the “gospel according to Peter,” since Peter was so close to the younger Mark.

Luke was closely associated to Paul. His gospel was written during his travels with the Apostle Paul, making Paul aware of it if not a major contributor. James was the half-brother of Jesus and a leader of the church of Jerusalem. Jude was also the half-brother of Jesus.