If the date for Christ’s crucifixion is around 30AD

And

MT was written in the late 50’s to early 60’s;
MK was written in the mid to late 60’s;
LK was written in the early 60’s
JN was written in the 70’s to the turn of the century.

Then it is correct to say that there was a delay in writing the Gospels; at least 25 years for MT, 30 years for MK and LK, and 35+ years for JN.

*all dates are estimates or approximations*

There are, however, some valid reasons for not providing a written Gospel early on:

The general population was not literate. There was not much, if anything, in their lifestyles that would demand them to be so. Also there were no printing presses so the dissemination of reading materials was very time consuming and very expensive. Which made reading and writing only something the elite would have access to, for the most part.

So if it was the goal of the disciples in the first century was to reach as many people as possible with the Gospel, it would have been counterproductive to write it down. Thus it was the speaking of the Gospel, or the oral proclamation of the Gospel that was most effective.

In Acts one can see that the church is quickly growing and spreading. There is no way to conceive this as happening if they had to wait to write and distribute scrolls or codices . And then, I suppose, teach everyone to read.

In any case, the time between Jesus’ crucifixion and the writing of the Gospels was not a static period. The Gospel was preached, disciples were made, and that happened over and over again. Thousands of times. The disciples, not just the 12, but all of Christ’s disciples would have heard the stories of Christ’s words and deeds many, many times. This is a very important point to remember.

Only when the Apostles were starting to die off – and it seemed likely that Christ was not coming back any time soon – did there seem to be any motivation to write the Gospels.

Now lets take a look at my question:

After Matthew wrote his Gospel, let’s suppose that it is read in a public meeting. What would have happened if Matthew had decided to change a few things here and there?

The people would have known if there were anything nefarious going on. Why? Because they already knew the Gospel story in side and out; they’ve heard it – as I said many, many times before – and Matthew’s Gospel would NOT have been accepted.

As I understand it one of the hallmarks of whether a book made it into the canon the fact that it was accepted by the church – i.e. the people. This is one thing I could point to as showing that the Gospel writers got it right; their writings were in harmony with what was already known. Also it is most probable that there were many eyewitnesses to the events and words of Christ who were still alive. The Apostles were not writing new stuff, so it could be checked.

Now if the Gospel writers had a penchant for changing anything one would have been the disciples being rebuked by Jesus or their “dullness” in understanding Jesus, or their attempting to position themselves for leadership. But all that “embarrassing” stuff in still there. This is an indicator that they were telling a true story.

Also all four gospels say that a woman discovered the empty tomb and were the first to learn that Jesus was alive. The significance of that? In those days, in that society, women were considered poor witnesses due to their “vanity and rashness”. They were so undependable that a testimony from one man equaled that of 100 women. Why would they leave that in if they were trying to make the story more believable? They obviously were more concerned with telling it like it happened.

Now I already accept the fact the early widespread distribution of NT books after they were written was a sort of control – i.e. any changes would have been noticed. Well that is exactly what the widespread distribution of the “oral proclamation” of the Gospel was – a control so any changes would have been noticed.

So the answer my question: “How do we know that the NT writers got it right before they sat down to write”, would be that the written Gospels were NOT the first time that story was told. The collective memory of the Christian community, who were very familiar with the Gospel by its constant retelling throughout the years and its confirmation by eyewitnesses who were still alive, served as the control for the Apostles accuracy.