Luke 24:37

Therefore I think it most natural that our Lord indeed instructed His disciples, that they should see Him in Galilee, but that He first presents Himself as they remained still in the assembly through fear. But afterwards when their hearts were strengthened, the eleven set out for Galilee. Or there is no difficulty in supposing that they should be reported to have been fewer in the assembly, and a larger number on the mountain. But persuaded by the example of their virtues, we can not believe that Peter and John could have doubted. Why then does Luke relate them to have been affrighted. First of all because the declaration of the greater part includes the opinion of the few. Secondly, because although Peter believed in the resurrection, yet he might be amazed when the doors being closed Jesus suddenly presents Himself with His body. Let us then consider how it happens that the Apostles according to John believed and rejoiced, according to Luke are reproved as unbelieving. John indeed seems to me, as being an Apostle, to have treated of greater and higher things; Luke of those which relate and are close akin to human. The one follows an historic course, the other is content with an abridgment, because it could not be doubted of him, who gives his testimony concerning those things at which he was himself present. And therefore we deem both true. For although at first Luke says that they did not believe, yet he explains that they afterwards did believe.

The disciples had known Christ to be really man, having been so long a time with Him; but after that He was dead, they do not believe that the real flesh could rise again from the grave on the third day. They think then that they see the spirit which He gave up at His passion. Therefore it follows, But they were terrified and affrighted, and supposed that they had seen a spirit. This mistake of the Apostles was the heresy of the Manichæans.