Peter, James and John recognized Him at the Transfiguration, even though His countenance was changed and his clothes purest white. Of course, the four of them were the only ones present until the vision of Moses and Elijah appeared.
Speaking of which, imagine that, as a pious Jew, you had heard all your life about these legendary men - but more than legends - chosen by God to shepherd His people and carry his word to them. For 1,250 or so years, their names had been on the lips of the devout.
All of a sudden, there they are! A few feet away - and no living person for over 1,000 years had seen them! Their appearance revealed only to the three chief Apostles. Is it any wonder that Peter was babbling at the sight? Wouldn’t we be utterly shocked - most likely speechless, as James and John were?
Back to the subject at hand: The two disciples en route to Emmaus, having spent perhaps the past three years with our Lord, failed to recognize Him either by appearance or by voice. Only in the breaking of the bread did they know Him.
In the upper room, when He appeared through the locked doors (or walls or ceiling or floor), they thought he was a ghost. Saint Thomas, who was ready to die with Him in Jerusalem, had to poke and probe those holy wounds before exhibiting credulity.
Only then were Thomas’ eyes and understanding opened. But, we have a preview of Him right now! He appears in the eyes and faces of those around us, of those in need, of those we love, of those who love us, and especially, in the faces of those whom we find it the most difficult to love.