In spite of a few conspiracy theories that state some, or even all New Testament books were written in the second century or later, we are 100% certain that the entire New Testament was finished by 96 A.D. (with 95 A.D. being the traditional date assigned to Revelation). Nevertheless, whether they were written in the first century or not, the Gospels themselves were actually written a lot later than the events they describe. Mark's Gospel was written around 50 A.D., followed by Matthew (55 A.D.), Luke (60 A.D.) and John (85-90 A.D.). Many sceptics seize upon this fact to claim that they may be unreliable. Enough time allegedly passed between the events described in the gospels and the time they were finally written down for both legendary embellishments and failures of the memory to affect the faith.
There are several major flaws with this argument, not least of which being it is circular. See, a critical doctrine in the Christian faith is that the Bible is inspired by God. That is, though it was written by men, it was ultimately inspired by God. Now, it would be circular reasoning to say that the Bible is inspired, and is therefore true, but it is equally circular to say that it isn't inspired, and therefore this objection is sustained. Put simply, if Christianity is true, the Bible is inspired, and if the Bible is not inspired, Christianity is not true. Sceptics who raise this particular objection must start with the assumption that Christianity is not true and the Bible is not inspired, therefore, since this is the very belief they are trying to establish, this is a circular objection.
But what makes this so much worse is that, even by secular standards, it is a flawed objection. First of all, it is not logic that would be applied to any other historical document. We might even point to Islamic history, which is not known from any contemporary source. Almost everything we know about early Islam comes from sources that were written over 100 years later. On top of that, ironically, there are some "other" gospels, such as the Gospel of Thomas, that these same sceptics often try to say belong in the New Testament, even though these are demonstrable forgeries written long after the Apostles were dead. If they're going to give later forgeries more credibility than contemporary, even eyewitness accounts, they have a problem.
So, evidently, this logic is not applied elsewhere. And for good reason: It makes no sense. The first problem is that even today, most people of the right age can remember things from about as long ago. The Holocaust, for example, is still within living memory, and some survivors tell their stories. Livia Bitton-Jackson is one of them. In 1997, her book "I Have Lived A Thousand Years" was published, in which she tells her story. What fool would have walked up to her and said "That was more than 50 years ago, therefore it's not true"? And yet, would this not have been the same logic that tells us the time elapsed between the evens of the Gospels and the authorship thereof is too great for them to have been reliable?
Furthermore, just as Holocaust survivors have a vivid memory of their experiences, so also did the Apostles have that. In fact, likely more so. These men lived in a culture that thrived on the power of human memory. Back then, they didn't have the internet. They didn't have computers. Even books were expensive to produce, and thus expensive to obtain. Our technology has weakened us. Personally, while my mother seems to have an insane ability to remember phone numbers, and even bank details, I remember only two phone numbers: My home landline, and my own mobile. Technology makes us sloppy. The lack of it made first century Jews considerably less so, with the most educated among them being the living equivalent of a library. They were able to memorise entire Scriptures, you don't think they could remember spectacular events from their own lifetimes?
We must also factor in repetition. While it may have taken longer to write the Gospels, the Apostles certainly didn't take as long to start preaching it. They dedicated themselves to diligently distributing the teachings of Christ from day 1, preaching the same message "in season and out of season" (2 Timothy 4:2) on a daily basis. What are the chances of forgetting something you teach every day?
Another thing we must account for is that the Gospels were almost certainly not the first, or the only reliable written accounts. In fact, ironically, many scholars posit the existence of the "Q" document, a hypothetical written source from which Matthew, Mark, and Luke may have drawn. This is one explanation for why these three gospels seem so remarkably similar. But of course, if this document does exist, it must have existed before the Gospels that allegedly drew from it. Furthermore, Luke tells us in his gospel that "many have taken in hand to set in order a narrative of those things which have been fulfilled among us, just as those who from the beginning were eyewitnesses and ministers of the word delivered them to us". It is likely that the fourfold Gospel just happens to be the only first century account that has survived. Whether the disciples kept individual records of certain sayings/works of Jesus, or even the mythical Q document existed, it seems unlikely the fourfold Gospel were the first written accounts, and wholly unreasonable to insist that they were.
And so we see that the time between the events of the fourfold Gospel and the time each gospel was written is, ultimately, irrelevant. By secular standards, it is unreasonable to say this time would have made a blind bit of difference to the historical accuracy of these accounts. When you add the Holy Spirit to the mix, whether as a known fact to the Christians, or even as just a possibility to the honest truth seeker, even the most reasonable of doubts becomes foolish. This leaves us with a very important question. If the New Testament is historically reliable, what does that mean for me?
First, it means that there is a God, and we are indeed accountable to Him. This is bad news, because none of us have fulfilled His standard of holiness. Quite the opposite, every single one of us has violated His law, earning for ourselves His eternal wrath. However, the New Testament also tells us that God has solved this problem for us. Though we deserve this wrath, He loved us, and so He sent Jesus, His Son, to walk among us as a man. Jesus was 100% God, and 100% man, and not once during His life did He sin. Because He never sinned, He never deserved punishment. Yet, He received punishment. He received it on our behalf. Jesus died on a cross, and was laid in a tomb. After this, He rose again. Those who confess that Jesus is Lord, and believe He rose from the dead, will be forgiven for their sins, and will one day be raised to life along with Him, to live eternally in the Kingdom of Heaven.