As the theory goes, the appearances were not really Jesus at all, but someone impersonating Him. This, skeptics will say, is evident because in some cases they did not recognize Him at first (or at all).
Three thoughts to consider...
The disciples were reluctant to believe in the Resurrection, were doubtful, and it would have been impossible to convince them unless it was really Him.
It would have been impossible to impersonate Christ’s wounds. This was Christ’s proof to Thomas that it was really Him.
What also needs to be taken into consideration is that the disciples were not looking for Him to be "risen from the dead", did not fully understand (Mark 9:32), and it was early morning, so perhaps the sun may have been bright when Mary mistook Him for the gardener (John 20:15), whereas distance may have been a factor to this event as well as when the disciples were fishing (John 21:4), and at times their inability to recognize Him was a phenomenon of His glorified body brought about by His own purposes:
Luke 24:16 – “But their eyes were holden that they should not know him.”
These men had traveled with Jesus for three years and it is incredible that anyone could have gotten away with an impersonation particularly due to their reluctance to believe. They were meeting in locked chambers in some instances, and He suddenly appeared in His glorified body. No one could impersonate such a miraculous act. I dive a bit more in detail on this topic in my critique of Islam and one of their possible views that Jesus did not die on the cross. Check it out, if you are interested in knowing more on this topic.