Were the disciples "seeing things?"
The hallucination hypothesis is plagued with problems. For one thing, the women didn't expect the resurrection. Their entire reason for visiting Jesus' tomb early in the morning was to anoint his dead body. Luke interviewed at least some of the women and reported that they were confused to find the tomb empty. (Luke 24:4) Mary Magdalene wasn't expecting to see Jesus. At first, she mistook him to be a gardener. (John 20:15). Luke reports that at least five women were involved, and we know that group hallucinations are extremely unlikely.
In addition, the disciples were probably not in a state of mind that made them prone to hallucinations. The apostles weren't like eager religious pilgrims who flock to holy sites hoping to see visions. They didn’t expect a miracle at all, let alone resurrection. To their embarrassment, the gospels make it clear that the disciples didn’t understand Jesus' somewhat puzzling predictions of his own death and return to life as one of impending bodily resurrection until after the event. Rather than anticipating the resurrection, they were grief-filled and afraid. (Matthew 26:56; John 19:38, 20:19). Messianic expectations in Judaism in the 1st-century didn’t include the resurrection of the messiah except in the general resurrection at the final judgment. (Daniel 12:2-3, Isaiah 26:19) Because of this, the disciples were initially also skeptical of others’ accounts of the empty tomb or of encounters with Jesus. (Luke 24:11)
But things get even worse for the hallucination hypothesis. To explain the facts, the hallucination theory would have to be applied to more than a dozen people at once (Luke 24:36-43). The plausibility of a collective hallucination is obviously inversely proportional to its level of detail. Given the level of multisensory details reported in cases like the one in Luke 24, the probability of mere coincidence that they all saw the same thing is ridiculously low.
Another reason complicates this problem: the hallucinations would have to be not only parallel but also integrated. According to the gospels, the risen Jesus interacted with his disciples in a number of ways, including eating food they gave him (Luke 24:41- 43) and Jesus cooking fish for them on the shore of the lake (John 21:1-14). In contexts like these, the disciples were interacting not only with Jesus but also with each other, both physically and verbally. It’s hard to believe that their parallel multisensory hallucinations were smoothly integrated. This would require a miracle in itself.
Lastly, these very detailed, parallel, integrated hallucinations had to be experienced repeatedly across 40 days during which the disciples were convinced they repeatedly interacted with Jesus here on earth. And then suddenly, the hallucinations stopped. Christ stopped appearing on earth. The visions of Peter and Cornelius in Acts, and even the appearances of Paul on the road to Damascus, are distinctly different from these encounters. The bodily appearances ceased after Jesus' ascension. Paul never claimed that Jesus ate fish or broke bread with him.
And for that matter, Paul's vision wouldn’t have to be just any hallucination, but a complex vision of the hated Jesus in glory, rebuking him. In addition, it is a weird kind of hallucination that is followed by several days of blindness, only to have one's sight restored after being prayed for. For all these reasons, the hallucination hypothesis is an utter disaster. The disciples were not deceived.
Process of Elimination
"Although at least a few if not all of Jesus’ disciples may have been in an emotional state that rendered them candidates for a hallucination, the nature of some of the experiences of the risen Jesus, specifically those that occurred in group settings and to Jesus’ enemy Paul, and the empty tomb strongly suggest that these experiences were not hallucinations.”
- Will Durant, an American writer, philosopher, and historian. Best known for his 11-volume "The Story of Civilization".