Is Jesus' resurrection appearance being doubted in Matthew 28:17?

Matthew doesn't explicitly tell us what was being doubted. But we get an indication of the most likely answer by reading what follows. Jesus' comments in verses 18-20 don't make sense as an attempt to persuade the people who are present that he had risen from the dead or that they were seeing the risen Jesus. Those subjects don't come up. But his comments do make sense as an encouragement to people who were doubting in the sense of lacking the confidence in him that they needed to proceed as they had to in that context. He reassures them about his authority and that he'll be with them. In other words, the doubt is about the implications of the resurrection, not the resurrection itself or this particular resurrection appearance. Jesus had just been put to death by his enemies, by means of a crucifixion arranged by the Jewish authorities and the Roman empire. He had risen from the dead, but a death, and a horrible one, was part of the process, along with a lot of other suffering. The people Jesus was addressing knew they were going out into a hostile world. In fact, the disciples' abandoning of Jesus in the face of such persecution at the time of Gethsemane is connected to this resurrection appearance in 26:31-32. It would make sense, then, for Jesus to address that sort of doubt in the context of the resurrection appearance anticipated in chapter 26. The worship mentioned in 28:17 and the activities of the Great Commission mentioned in the verses that follow were some of the appropriate ways to proceed, and they should have proceeded with confidence, "but some doubted". The doubt isn't about whether Jesus rose from the dead or whether he was appearing before them on this occasion, but, instead, was about how to proceed. He was removing their doubts and building up their confidence in verses 18-20. Those closing verses make less sense if the doubt in question was about whether Jesus rose from the dead or whether he was appearing to them.

You Can Emphasize The Resurrection Without Isolating It

Follow Peter's example in Acts 2, where he highlights Jesus' resurrection while also addressing prophecy fulfillment, Jesus' miracles other than his resurrection, and the miracles of the apostles. And Jesus, the prophets, and others did the same before Peter. Make much of Jesus' resurrection, but bring up other lines of evidence for Christianity as well.

The Level Of Detail In 1 Corinthians 15:6

Jesus' resurrection appearance to more than 500, mentioned in 1 Corinthians 15:6, tends to get underestimated in our day. But there's a lot to commend it and warrant assigning the passage more significance than people often do.

One of the reasons why the passage should be held in higher regard is the level of detail it includes about significant issues. Paul is briefly summarizing several of Jesus' resurrection appearances, yet a series of important details about the appearance under consideration are included even in that brief summarizing context. Paul refers to the relative chronology of the appearance ("After that"), the number of people involved, saying "more than" instead of just leaving it at a rough estimate of 500, specifies their gender ("brethren"), recognizes the significance of their having seen Jesus "at one time" and the importance of mentioning that detail, and followed their lives since the time of the appearance enough to know that "most" are still alive and the value of their still being alive. (See here regarding the likelihood that some non-Christians were present during the appearance.) Paul not only experienced a resurrection appearance himself, but also had a lot of interest in and knowledge about the appearances to others. And the details he shows interest in in 1 Corinthians 15:6 reflect well on him, since they're such significant ones.