The Gospels were written decades later by people who weren't there, so they're unreliable

The fair point

The Gospels weren't written the day after the events. Most scholars date Mark to around AD 65-70 and John to around AD 90-100. Their authors had theological purposes, and there are differences in detail between them. Bart Ehrman, a respected New Testament scholar, has written popular books like Jesus, Interrupted highlighting these issues. These are real questions.

By ancient standards, that's early

Compare the Gospels to other ancient biographies. The earliest surviving biographies of Alexander the Great, by Arrian and Plutarch, were written more than four centuries after he died, yet historians consider them generally reliable. The Gospels were written within the lifetime of people who knew Jesus. And Paul's letters are even earlier. In 1 Corinthians 15, written around AD 55, Paul passes on a creed about Jesus's death, burial, resurrection and appearances that most scholars date to within a few years of the crucifixion.

Eyewitness testimony

Richard Bauckham, in Jesus and the Eyewitnesses, argues that the Gospels are closer to eyewitness testimony than many scholars have assumed. He points to the way named individuals appear in the texts, possibly as sources, and to how ancient historians valued living witnesses. You don't have to accept every detail of his case to see that the telephone-game picture is too simple. Oral cultures were often very careful about preserving important teachings, and the communities included people who would remember.

What about the differences?

The Gospels do differ in details, such as how many angels were at the tomb. But historians expect independent accounts to differ in minor details; perfect agreement would suggest copying. What matters is that they agree on the core: Jesus taught, was crucified under Pontius Pilate, was buried, and his tomb was found empty, with followers convinced they'd seen him alive. Even many skeptical scholars accept much of that.

Corroboration

Non-Christian sources like the Roman historian Tacitus and the Jewish historian Josephus mention Jesus and his execution. Archaeology has confirmed many details of the Gospels' setting, such as the existence of Pontius Pilate and the pool of Bethesda.

A fair concession

Historical evidence can't produce certainty, and scholars disagree on how much of the Gospels is precise history. Reasonable people can weigh things differently.

How to explain this to a friend

It helps to compare the Gospels with other ancient sources your friend might trust, such as what we know about Alexander the Great or Julius Caesar. Then point out how early the Gospels and Paul's letters are by comparison. You don't need to claim they're flawless, only that they deserve to be taken seriously as historical evidence, and that the core facts are widely accepted even by skeptical scholars.

The bottom line

The Gospels were written within roughly 30 to 65 years of Jesus, while eyewitnesses were still alive, which is remarkably early by ancient standards. They're worth taking seriously as historical sources. For the bigger picture, see Is Christianity true?