There is a popular myth that the earliest writings about Jesus came a generation or two after his death and therefore can’t be all that accurate. This is simply not true.
The earliest writings that reference Jesus and early Christianity are those of Paul written between 48-60 AD/CE -beginning roughly 15 years after the death of Jesus. In I Corinthians he writes about the Last Supper and the Crucifixion and Resurrection of Jesus. In Chapter 15 he names Peter, James the brother of Jesus, and the Apostles, plus himself and 500 others as those who had seen Jesus after his resurrection. In Galatians he claims that he received the Gospel directly from the resurrected Jesus.
The Synoptic Gospels: Matthew, Mark, and Luke-Acts were written before 70 AD/CE. Probably between 55-66 (22-32 years afterwards). Internal references and evidence pretty much precludes a later date considering the Jewish War (66-70) and the changes following. 30 plus years may sound like a lot for some but I was married to my wife over 47 years ago and I can still recall the events of our wedding and honeymoon like it was yesterday. Plus what I don’t remember my wife does.
Also, most Bible scholars believe that the 3 synoptic Gospels contained earlier common and diverse eye witness source material that was probably circulated orally. In the first century, memorization and oral recall of large amounts of material was quite common. Most Judean families maintained a lengthy genealogy in memorized oral form. Also, most Rabbis in the first century accurately memorized the entire Torah (first 5 books of the Bible) and the more respected ones could substantially recite the Prophets and the Writings –the entire Tanakh –what Christians call the Old Testament.
As long as most of the Apostles and early Christian leaders were all together in Jerusalem and neighboring areas, there was little perceived need for a written Gospel. After the movement began to spread to Asia Minor (45 AD) and then on to Greece and Rome (48-60 AD) and all across the Roman Empire, the need became obvious and more permanent written records of the oral traditions were made.
The Gospel of John did come much later and was written around 90 AD/CE by John the Beloved Apostle who lived a long life and ministered into the 2nd and third generation of Christians. Nevertheless he was a first-hand witness to the life and ministry of Jesus and was able to add important stories and teaching in his gospel that was left out of the other three.