John's Gospel has quite a different flavour to the other three, and it has unique stories about Jesus. It's also the Gospel that's most explicit about Jesus being God incarnate. So if you ever have detailed debates on this subject, you're likely to come across arguments that John isn't reliable.
I was fed that information before I became a Christian, from I don't know where. I thought the other Gospels of Matthew, Mark and Luke told how Jesus just taught nice things, whereas the Gospel of John was a 'Christianised' Gospel that only reflected what Christians wanted Jesus to do and say, rather than what Jesus really did do and say.
However, many scholars have provided good arguments to challenge such ideas. Here are a few:
John was written within a lifetime of Jesus' death and resurrection
For some time, John was thought to be a much later document than the other three Gospels – written in the second century, and therefore more than 70 years after the death and resurrection of Jesus. However these kind of arguments often depend on the easily debunked idea that the belief in Jesus as God didn't exist in the Church at first but only developed gradually over time. This ignores the fact that some of the earliest New Testament letters clearly describe Jesus as God (eg Philippians 2:6-11, Colossians 1:15-20).
However modern scholars say John was more likely written between 80 and 100 AD – a time when there would still be people alive who had lived alongside Jesus and witnessed the events first hand.
The author of John's Gospel was a disciple close to Jesus
The 12 disciples were the closest friends of Jesus during his ministry, so what they said about him has to have weight. However, some argue the Gospel of John wasn't written by the apostle John, who was one of the 12. Yet the earliest church writers, such as Irenaeus, said that it was. Craig Blomberg gives five reasons why it's most reasonable to agree with them, such as: there's a strong case that the author was Jewish, from Palestine, and an eyewitness of the events.
The 'disciple Jesus loved' mentioned several times in the narrative as an eyewitness, is clearly stated to be the author of this Gospel (21:20-24), and was one of the 12 (13:23). So even if it wasn't John, it's still someone very close to Jesus. So how do we know for sure whether it was John? Blomberg argues that a hint is that the Gospel calls John the Baptist simply 'John', unlike the other Gospels. With two main characters in the story having the same name, a reasonable explanation for John's gospel not distinguishing between them is that John was writing the Gospel for people who knew him, and so he didn't need to explain or make that distinction. Therefore it's reasonable to trust the earliest Christians' record that it was John the apostle who wrote this Gospel.
There isn't a contradiction between John and the other three
Yes, John has a different style and flavour to the other 'synoptic' Gospels – John doesn't have parables, and the way he records Jesus' discussions and his literary style are very distinct. There are different stories included, and John doesn't use some key stories that are used in the other three. Some argue that this means it's less reliable. Yet John says in 21:25 that "Jesus also did many other things. If they were all written down, I suppose the whole world could not contain the books that would be written", which suggests that the disciples had a lot to choose from, which would make sense after hanging out with him for three years. And it seems reasonable to believe that different eyewitnesses would just record events in different ways.
John was concerned with history, not just theology
John may have wanted to emphasise some theological points, but that doesn't mean he wasn't intending to record history or what Jesus said accurately. To support the idea that the writer was an eyewitness, and recording events that actually happened, Blomberg argues that John gives many small, precise details and facts, and there are unexplained sayings, and information that is challenging or contradictory and so unlikely to be invented. Plus the writer of the Gospel explicitly says that he is recording actual events that he has witnessed (21:24).
John and the other Gospels accept Jesus as God
One of the most significant reasons for people to object to John's Gospel, is the idea that the other Gospels don't say that Jesus is God incarnate. This is a big issue for Muslims, Jehovah's Witnesses and other faiths.
Yes it's true that John's records of Jesus' words can be more explicit about his identity – such as 1:1-3 ('The Word was God'), 20:28 when Thomas exclaims 'My Lord and my God', and in 8:58 where Jesus says "I tell you the truth, before Abraham was even born, I am!" which references Exodus 3:14.
However, a strong case comes from the other Gospels. They show that Jesus accepted worship (Matthew 2:11, Matthew 28:9, Luke 24:52), forgave sins (Mark 2:5) and identified himself as Judge (Matthew 25:31-46), and the Messiah sitting at the right hand of God (Mark 14:61-62). The religious leaders called this blasphemy, because they knew what this meant: Jesus was claiming to be equal with God (John 10:33, 5:18). As described above, early Christian writings such as Philippians and Colossians describe Jesus as equal to God. So John's Gospel is in harmony with our evidence of other early church beliefs.
Many commonly heard objections to John rely on presumptions and biases. There are good arguments to say that the Gospel of John was written by one of the people who knew Jesus best, was closest to the events, and who wanted to record them accurately.
Heather Tomlinson is a freelance journalist interested in investigating and communicating the Christian faith. She blogs at www.heathert.org and is on twitter @heathertomli