If four journalists were assigned to cover the same news story, we would expect four similar, yet slightly different perspectives. The basic facts would be the same, but the details would depend on their vantage point, research, writing style, and audience.
The same is true of the four Gospels. They all testify to the life, death, and resurrection of Jesus Christ. But some details are unique to each gospel writer. At first glance, they can even appear contradictory. Take the Transfiguration, for example.
Matthew 17:1 says “And after six days Jesus took with him Peter and James, and John his brother, and led them up a high mountain by themselves.”
Likewise, Mark 9:2 records “And after six days Jesus took with him Peter and James and John, and led them up a high mountain by themselves.”
Luke 9:28 however, says “Now about eight days after these sayings he took with him Peter and John and James and went up on the mountain to pray.”
Is this a mistake? No. In fact, this minor difference actually confirms the reliability of the Gospels. Let me explain.
Luke's word 'about' (Gk. hosei) means 'as if, nearly, something like' (Vine's Dictionary). The author admits he is only giving a ballpark figure here. Unlike Matthew, Luke was not an eyewitness. During his research, perhaps it was difficult to nail down the exact timing of this event. It could've been six, seven, eight, or maybe even nine or ten days later. Six certainly falls within the range of 'about eight.'
But here’s what’s most significant. Luke's account of 'about eight days' suggests he was not using Mark as a source. This verse actually helps disprove Markan Priority, or the theory that Matthew and Luke leaned heavily on Mark's gospel to pen their own.
It would have been easy for Luke to just copy the number six if he had Mark's gospel in front of him. But it seems that each gospel writer gave their own account through personal testimony, painstaking research, and the inspiration of the Holy Spirit.