John Calvin Commentary Luke 24:31

"And their eyes were opened, and they knew him; and he vanished out of their sight." — Luke 24:31 (ASV)

And their eyes were opened. By these words, we are taught that Christ did not undergo any metamorphosis, or variety of forms, by which he might deceive men's eyes (as the poets pretend concerning their Proteus,). On the contrary, the eyes of the onlookers were mistaken because they were hindered. This is just as, shortly afterwards, he vanished from the eyes of those very persons, not because his body was inherently invisible, but because God, by withdrawing their visual power, blunted their sharpness of sight.

Nor should we wonder that Christ, as soon as he was recognized, immediately disappeared. It was not beneficial for them to see him any longer, lest, as they were naturally too much attached to the earth, they might desire to bring him back to an earthly life again. Therefore, insofar as it was necessary to assure them of his resurrection, he made himself visible to them; but by his sudden departure, he taught them that they must seek him elsewhere than in the world, because the completion of the new life was his ascension to heaven.