A saying we would all do well to abide by is "if it isn't broken, don't fix it". The general implications, of course, are that life runs smoother if we spend more time fixing the problems we have than inventing problems to fix.
This sound advice is often rejected by anti-Christian scholars. The Bible has a long and rich tradition detailing its own origins. With a few exceptions, such as the books of Job and Hebrews, we have a rather solid idea of who wrote each book, and when they did so. However, this is rarely convenient for anti-Christians, who, seeking to contradict these books, also seek to re-write established history just to support their own conclusions.
Ultimately, as is the nature of Biblical skepticism, no book is safe. However, for sake of this article, I will briefly focus on just three (or, technically, 7): The book of John, the book of Daniel, and the Pentateuch.
John
Being both a close follower of Jesus during His ministry, and the only disciple who dared approach Him as He hung on the cross, John's Gospel cannot have been written much later than the late 90s A.D. But if you doubt its authenticity, it's just as easy to doubt its authorship date. And if you doubt its authorship date, you can of course doubt its authenticity.
In a vain attempt to enforce such doubts, Liberal German scholars used to speculate that John was actually written significantly later, in the second century, much like many pseudepigraphal writings such as the Gospel of Thomas.
A simple piece of evidence against this claim is that there has never been any evidence for it. The traditional authorship dates of all Biblical works have pretty much always stood mostly unopposed. The only "real" reason to doubt them is if they don't fit in with your worldview, but this is circular reasoning, no better than citing 2 Timothy 3:16 as proof that the Bible is divinely inspired.
Of course, as a Christian, I believe that is the case, and if I was to debate this issue with a fellow, misguided Christian, that would be sufficient. But the good thing about truth is you don't have to believe it in order to discover it. Just as you cannot fly by rejecting gravity, you cannot make John a pseudepigraphal writing just by claiming it is so.
The result of the simple fact that John wasn't written in the 2nd century is the so-called P52 Papyrus, also known as St. John's Papyrus. Regarding this fragment, Dan Wallace writes "This tiny fragment of John’s Gospel rocked the scholarly near-consensus on the date of John, for it is impossible for a copy to be written before the original text is produced. It effectively sent two tons of German scholarship to the flames. As one wag put it, “This manuscript must have been written when the ink on the original text was barely dry."
Although only fragmentary, being the size of a credit card, the P52 Papyrus is clearly an early copy of the Gospel of John, being written, at the latest, 175 A.D. This makes the more traditional authorship date for John's Gospel far more reasonable.
Daniel
In 2 Peter 1:20-21, Peter tells us, "First of all, you should know this: No prophecy of Scripture comes from one’s own interpretation, because no prophecy ever came by the will of man; instead, men spoke from God as they were moved by the Holy Spirit." If this is so, we would expect the prophecies in the Scriptures to be far beyond, for example, predicting who will win a game or an election, or what the weather will be next Saturday.
One of the best Scriptures that fulfills this in astonishing detail is the book of Daniel, which was written around 530 - 550 B.C. "Not so!", saith the skeptics. It was actually written in the 2nd century B.C., after the events it describes. It's not prophecy, it's commentary.
Interestingly, this is not a modern theory. It was initially posed in the 3rd century by a Greek pagan named Porphyry. But this theory, interestingly enough, falls on its own sword. One argument used in its defence is that Daniel uses Greek words, which, by the estimation of certain conspiracy theorists, shouldn't be possible if the traditional dates are true, since Alexander the Great had not yet conquered Babylon.
However, the three Greek words used in Daniel are actually transliterations of instruments, which were in use in Babylon at the time. Furthermore, two of the three are found written in Babylonian documents older than Daniel. This means Daniel is written in the exact languages we would expect if the traditional dates are true, and not if the fake dates are.
Moses
Moses is perhaps the most prolific author of the Bible, with even Jesus setting His divine feet down on Moses' foundation. This, again, should be enough for misguided Christians to accept that Moses did, indeed, write the books ascribed to Him. But for anti-Christian skeptics, a more plausible explanation is the Documentary Hypothesis, a conspiracy theory in which the Pentateuch, rather than being the work of Moses, is a compilation of various writings from different sources, ultimately coming together in the time of King Josiah in the 7th century B.C.
This is perhaps the most laughable "theory", owing to the fact it is so devoid of rational thought, it cannot be kept straight. It has been revised many times since its inception, and I'd be interested to compare the writings of its remaining proponents to see just how often they contradict each other. But as fun as it is watching the devil trip himself up, we don't need conspiracy theorists to refute their own Documentary Hypothesis.
Of course, in reality, Moses did not write the whole Pentateuch. Deuteronomy contains an account of his death and burial, which he of course was not able to write about personally. The most reasonable explanation is that Joshua, Moses' immediate successor, and the author of the subsequent book, wrote that part.
However, both internal and external evidence utterly refutes the possibility that the Pentateuch is the work of many authors. At a later date, I intend to write a larger article refuting this conspiracy theory, but for now, the only evidence I want to focus on is chiasmi. A chiasmus is a figure of speech where two parallel terms are laid out, with the second set inverted. A famous example would be "ask not what your country can do for you, but ask what you can do for your country".
The Pentateuch is filled with these, which is to be expected. The Jews, much like other ancient cultures, were far more adept at memorising oral traditions than reading their written ones. Both low literacy rates, and the expense of copying or obtaining a written record, meant older traditions had to be memorable.
In his work "The Inspiration of the Pentateuch", M.W.J. Phelan notes one such chiasum in the whole account of Noah, showing without reasonable dispute that the whole account is the work of one author.
The Documentary Hypothesis is, to say the least, the most ridiculous of all attempts to re-arrange Biblical authorship dates, with internal evidence like chiasmi being a small portion of evidence against it. There can be no reasonable doubt that Moses did, in fact, pen the Pentateuch around 1400 B.C.
Conclusion
These are just a few attempts anti-Christians make to alter the traditional Bible authorship dates, with a brief look at evidence for the more traditional view, but there is far more that could have been mentioned. No book is safe from conspiracy theories, and no conspiracy theory is safe from overwhelming evidence to the contrary. With this in mind, one must ask, why do anti-Christians try so hard?
The simple answer is because although the traditional dates pose no threat to the objective thinker, they do cause a major headache for anyone who would seek to oppose God. Altering the dates allows unbelievers to erase foundations, and ignore evidence. If prophecies are, in fact, commentaries, there is nothing spectacular about them. Anyone can write about what happened in history. If the Gospels were written later, there was time for legends to creep in, for theology to develop, and for real witnesses to simply die off. And if Moses is some mythological figure, then for Christ to base His claims upon his works is just one more proof that He is not the Messiah.
As time goes on, we will see more and more baseless claims made against the Bible, but with the evidence we have even now, we can happily dismiss them, knowing, as Peter said, that Scripture is not a work of mere men, but of the Holy Spirit, who guided His people to write His words. Prophecies, like those in Daniel, came true precisely because God knows the end from the beginning. Moses, not fully understanding what God showed him, really did foretell of the coming Christ. And that Christ really was witnessed by John, and many other authors of the New Testament. There is no reason to doubt this, but the reasons for accepting it go beyond just "it's true".
See, the Bible is not just a historical record, foretelling history in advance and recording it after the fact. It is also a message from God Himself, telling us both who He is, and how we relate to Him. The sad fact of the latter is that we do not do so very well. Rather than obey Him, as we should as His creations, we have all gone astray, offending His Holy character. As a result, we owe Him a debt we cannot pay, being destined for His everlasting wrath.
But it is not His will that any should perish. Out of love, He sent Jesus to live as a man in this world, to be tempted in all things as we are, yet without ever sinning. Yet, He suffered the full wrath of God owed for sin, allowing us to instead be rewarded for His righteousness. Eternal life, and an inheritance in the Kingdom of God, is available completely free for all mankind. There is but one condition: Faith. We must confess Jesus as Lord, believing in our hearts God raised Him from the dead. Then we shall escape sin and death, and when this life is over, we will reign with Him forever.