Bruce Metzger, Princeton University, puts it well when He said,  “the cannon is a list of authoritative books more than it is an authoritative list of books.”

What he meant by this statement is that Christians did not determine what should be in the Bible. Instead, Christians discovered what books were inspired and only put those in the Bible. 

They did so by asking the following questions:

• Was it written by an acclaimed prophet of God?
• Was the writing confirmed by the acts of God or by someone who was an established eyewitness? For example, Paul, who witnessed the resurrected Jesus and performed miracles, endorsed the books Luke who had interviewed multiple eyewitnesses. 
• Was it accepted by the people of God?

Further, the approved books were the ones that met the following criteria, for example. 

• The acts performed were cited / recorded during the lives of the apostles. 
• They the books were quoted as authoritative and unique by the early church fathers. 
• The books were collected in one initial volume rather than scattered manuscripts put together. 
• They were publicly read and expounded. They were not hidden books used only by a specific group. 
• Commentaries were written on these books, which means that they would have had to been thoroughly studied.
• There were also testimonies from opponents who even admitted that the disciples wrote the gospels. 

All the books of the Bible meet one or more of these criteria’s. 

In addition, people question recent discoveries of scripture and wonder why the books have not been included as part of the Bible. Famous examples of these books include the “Gospel of Thomas” and “Gospel of Judas.” 

However, these books are proven to be written by forgers who put the names of the apostles on it to try and authenticate their writings.

Further, these discovered books were also written during the second century and not within the lifetime of the apostles.

Lastly, when comparing these books to the above criteria’s they would fail in being classified as an authoritative scripture to be canonized.

We must note that if God is the divine author of the Bible, He would surely have ensured that His word is preserved by the power of the Holy Spirit. 

1 Peter 1:24-25 (NKJV)

24. because “All flesh is as grass, And all the glory of man as the flower of the grass. The grass withers, And its flower falls away,

25. But the word of the Lord endures forever.” Now this is the word which by the gospel was preached to you.