Ambrose of Milan — as excerpted in the Catena Aurea on Luke 24:36-40

“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.”