Those who hold that the Bible contains neither errors nor contradictions typically explain away this discrepancy by saying that Jesus started out carrying the cross alone, and later, Simon of Cyrene was pressed into service, thus eliminating the contradiction. Unfortunately this kind of harmonizing also eliminates a clear understanding of John's unique theology.
Let John be John, I say. And let each of the Bible's writers speak for themselves. They all fall within the spectrum of orthodoxy, but each proclaims the message in their own way. Rather than explain away their contradictions, let us attend to them and so learn each writer's unique perspective.
A few days ago, a friend emailed me a link to this article. It seems that Colin Humphreys, a physicist from Cambridge has written a book in which he claims to have resolved another discrepancy between John and the Synoptics. In Matthew, Mark and Luke, Jesus' last supper with his disciples is a Passover meal that apparently takes place on a Thursday. In the Gospel of John, Jesus is actually crucified on Passover, which means that the Last Supper must have taken place a day earlier.
In all fairness I have to say that I have not read Humphreys' book and honestly, don't think I will. So, I am not in a position to assess the strength of his arguments. According to the article referenced above, Humhreys resolves the difference between John and the Synoptics by arguing that Jesus used a different calendar than the one that “was in widespread use at the time of his death....” By that calendar, Passover would have fallen on Wednesday.
This may account for a one day difference in the date of the last supper, but it does not explain the three year difference in John's account of Jesus chasing the money-changers out of the temple as compared to the account in the Synoptics.
Nor does it explain some other, significant differences between John's account of the last supper as compared to those accounts in the Matthew, Mark and Luke. For instance, in John's Gospel there is no mention of the bread and wine that Jesus shared with his disciples; there are no “words of institution.” Instead, John's Jesus gives a lengthy “farewell discourse” and washes his disciples' feet. In short, there is no indication in the Gospel of John that the Last Supper was a Passover meal.
In John's Gospel, Jesus is referred to as the “Lamb of God” (John 1:29, 36) None of the other Gospels uses this title. The exact meaning of the term is hard to pin down (that was probably John's intent) but it is a symbol of sacrifice and salvation and echoes the account of the Passover in Exodus 12. That Jesus, the Lamb of God, would die on the Passover, at the same time that lambs were being slain in the temple at Jerusalem is symbolic, significant and fraught with meaning.
Some may want to explain away every contradiction in the Bible. Not me. John was not writing a history; he was proclaiming the gospel. Rather than conflate the four Gospels and lose their unique character, I think we should let each speak for itself.
The Lamb of God graphic came from good old wiki.