Since men hate truth, a vast array of conspiracy theories have surrounded Christianity literally since its inception. A conspiracy theorist's best friend is time. Any amount of evidence can erode given enough time, and of course it's been a rather significant amount of time since the time of Christ. Thus, conspiracy theorists are able to produce more elaborate stories than just "the disciples stole the body".
In Christianity, the Bible is the supreme authority. It not only contains all things needed for faith and practice (2 Timothy 3:15-17), but is also the historical foundation for the faith itself. The New Testament especially was completed in the first century, written by eyewitnesses and their contemporaries. Although Luke never actually saw the events described in his Gospel, he sought to write an orderly account for Theophilus. Thus, he investigated the events carefully. John, by contrast, was a close disciple of Jesus. The Gospels they wrote, along with the rest of the books of the New Testament, are solid history. We can be sure that Jesus lived, died and rose just based on the New Testament.
But what if that's not true? What if the books of the New Testament, such as the Gospel of John, were written long after the events they purportedly describe? Enter liberal German scholars, who for a long time claimed exactly that (and of course, many still do). According to legend, the Gospel of John is actually a second century forgery that was merely attributed to John decades later.
Attempts to push forward the authorship dates of books is not a new practice. For example, a third century pagan named Porphyry (por-fer-ree) is one of the earliest proponents of a similar attempt to post-date the book of Daniel, a book which so clearly prophesies what are now historical facts that the only possible way to claim it is not prophetic is to claim the book was actually written after the fact. But what evidence did Porphyry give to defend his claim that Daniel was written in the second century B.C. instead of, as the historical record shows, 530-550 B.C.? None, but of course, it contains prophecy, which we "know" is impossible, so it must have been written later.
You can see the folly of this reasoning. Porphyry posited a made up history simply because the history that exists is problematic to his religion. If Daniel was written around 530-550 B.C., and yet so accurately predicted the history that would happen long after Daniel died, that reasonably points us to the belief that Daniel worshiped a very real God. This is the same motive modern sceptics have for accepting the later date for Daniel today. It's not that the evidence compels us to accept the later date, it's that the actual date compels us to draw conclusions we don't want to draw.
The same is true for the Gospel of John. There is not now, nor has there ever been any good reason to believe any New Testament book was written later than 95 A.D. (when Revelation is believed to have been written). There is no evidence within the texts to suggest they were written later, there is even evidence within the texts that support the traditional dates (and in some cases, even support earlier dates), the whole idea that the book of John was written by someone other than John in the second century is utterly baseless.
But we're not just talking about an absence of evidence for the later dates here. There is positive evidence that the Gospel of John must be old. This evidence comes in the form of a small fragment of papyrus, named P52 (a.k.a. St. Johns Fragment). This fragment is the oldest fragment of John we have today, dated between 90 A.D. - 175 A.D. To quote Dan Wallace, "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.”"
P52 is only the size of a credit card, yet on its own, it is enough to topple a firmly entrenched conspiracy theory. In 1 Corinthians 1:26-29, we are told that God uses the foolish things of the world to put the wise to shame, the weak things to shame the strong, the despised things, and things which are not, to bring to nothing the things that are.
From beginning to end, we see this play out. From one man, He raised a nation. He sent a cowardly stutterer to free His people from Egypt. With one shepherd, He killed a giant. With one baby, He struck terror into the heart of a tyrant. Through His death on a cross, He conquered sin. He chose women to bear first witness to His resurrection. Today, He uses tiny fragments of paper, thrown out into an Egyptian rubbish dump, to overthrow so-called "scholarship" that denies all of the above. Ladies and gentlemen, the Lord God of Israel is too glorious to oppose! There is no sense in rebelling against Him! Repent and believe, or rest assured you will meet no better a fate than the conspiracy theories that set themselves against Him. Jesus rose, and He rose for you. If you confess with your mouth that He is Lord, and believe in your heart that He rose for you, you will be saved.