Theophylact of Ohrid

But since this word did not quiet the agitation of their souls, He shows them in another way that He is the Son of God, who knows the hearts.

Commentary on Luke ↗

Ambrose of Milan

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. …

Catena Aurea by Aquinas ↗

Bede

They were troubled and terrified, thinking they were seeing a spirit. This is what the Manichean heretics suspect and believe about Christ, that He was not true flesh, but a spirit. This first thought arose in the hearts of the apostles. And indeed those Manicheans never believed that Jesus was a man. However, the disciples knew the man with whom they had conversed for such a long time. But after He died, could they believe that what they knew could be raised again if it could die? …

On the Gospel of Luke ↗

Bonaventure

For which reason he adds: But troubled and frightened, they supposed that they were seeing a spirit. They were frightened because they had seen him dead on the cross, and because they feared lest "Satan should transform himself into an angel of light," and because he had come with the doors shut: whence John 20: "When therefore it was late on that day, the first of the week, and the doors were shut, Jesus came and stood in the midst and said to them: Peace be to you." But in this there seems to be a contradiction between Luke and …

Commentary on Luke, Chapter 24 ↗