There are two answers to that question. The short answer, and the easiest to defend, is also the most obvious. God did. The Bible says so.
“For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.” (2nd Peter 1:21)
I say that is the easiest to defend because Christians don’t need much more evidence than that. The Bible is a living Book to those who are indwelt by the Holy Spirit.
To a Christian, the mere fact that there are people who devote their lives to arguing its Authorship is evidence of its Divine inspiration. It makes perfect sense to a Christian—it makes no sense at all to an unbeliever.
While it is a totally unsatisfactory answer to the sceptic, 1st Corinthians 1:18 proves itself to the believer every time he picks up the Book and ponders its truths:
“For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.”
Frankly, I don’t think it is possible to convince a sceptic by debating the truth of the Scriptures vs. their chosen ‘truths’—it would be like debating whether something was red or mauve with a person blind from birth.
It takes a spiritual ‘operation’ to remove that blindness, but we can only point a person in the direction of the Surgeon. After that, they have to request the ‘operation’ for themselves.
But the Bible’s Authorship is proved by its very existence. There is the testimony of the forty different individuals chosen by God to record His Word. There are the acts of the Apostles;
2nd Peter 1:16explains;
“For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of His Majesty.”
The Apostle Luke begins his testimony to Theodophilus;
“Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us, Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the Word.” (Luke 1:1-2)
To deny the historical truth of the eyewitness testimony of the Apostles is tantamount to denying the historical accuracy of the eyewitness testimony of the witnesses to the assassination of Abraham Lincoln.
Each of the Apostles was an apostate Jew in the eyes of their friends and families. They were ostracized, insulted, beaten, run out of town, arrested, imprisoned, and generally hounded everywhere they went. Each of them was given an opportunity to save his own life by renouncing his testimony of Jesus.
And with the exception of the Apostle John, every single one of them chose a brutal, torturous death, instead. (The Apostle John was tortured by being boiled alive, but somehow survived and was exiled to the Island of Patmos. He was later freed and returned to serve as Bishop of Edessa in modern Turkey. He died as an old man, the only apostle to die peacefully.)
The skeptic denies their eyewitness testimony, but fails to give any reasonable explanation for why. Why would they all accept a life of misery and deprivation, culminating in a torturous death, just to spread a myth?
Does it seem reasonable that twelve guys would sit around a campfire and make up a story that ruined their lives (in the natural) just so they could be known by their first names 2000 years later?
Nobody denies the accuracy of Plato’s writings. Or Tacitus. Or Homer. Or Suetonius. Or Flavius Josephus (except the part where he refers to Jesus as an actual historical figure).
Before the discovery of the Dead Sea Scrolls our earliest Hebrew copy of the Old Testament was the Masoretic text dating around 800 A.D. The Dead Sea Scrolls date around the time of Jesus copied by the Qumran community, a Jewish sect living around the Dead Sea.
We also have the Septuagint which is a Greek translation of the Old Testament dating in the second century B.C. The oldest existing original manuscript of a New Testament book dates to 125 A.D. and was found in Egypt, some distance from where the New Testament was originally composed Asia Minor). In all, there are more than 24,000 ancient manuscripts against which to compare our modern Bible.
The number of manuscripts is astonishing, when compared to other universally-accepted ancient historical writings, such as Caesar’s “Gallic Wars” (10 Greek manuscripts, the earliest 950 years after the original), the “Annals” of Tacitus (2 manuscripts, the earliest 950 years after the original), Livy (20 manuscripts, the earliest 350 years after the original), and Plato (7 manuscripts).
New Testament manuscripts agree in 99.5% of the text (compared to only 95% for the Iliad). Most of the discrepancies are in spelling and word order.
A few words have been changed or added. There are two passages that are disputed but no discrepancy is of any doctrinal significance. Most Bibles include the options as footnotes when there are discrepancies. How could there be such accuracy over 1,400 years of copying?
1)The scribes that did the copying had meticulous methods for checking their copies for errors.
2) The Holy Spirit made sure we would have an accurate copy of God’s word so we would not be deceived.
Skeptics, liberals, and cults and false religions such as Islam that claim the Bible has been tampered with are completely proven false by the extensive, historical manuscript evidence.
But it doesn’t matter. The sceptics continue to assault the Bible on any and all fronts, applying the most unreasonable standards for accuracy imaginable.
They hate it, and they can’t even explain why. That is also, to the Christian, evidence of its Divine Origin.
That hatred is so blind, so unreasoning, and so irrational that it cannot be explained in any other way.
“Blessed are ye, when men shall hate you, and when they shall separate you from their company, and shall reproach you, and cast out your name as evil, for the Son of Man’s sake.” (Luke 6:22)
“Marvel not, my brethren, if the world hate you.” (1 John 3:13)
“Knowing this first, that there shall come in the last days scoffers, walking after their own lusts, And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation.” (2nd Peter 3:3-4)