72 Again, if Mark tells us that the Lord appeared to these persons in another form, Luke refers to the same when he says that their eyes, were holden, that they should not know Him. For something had come upon their eyes which was suffered to remain until the breaking of the bread, in reference to a well-known mystery, so that only then was the different form in Him made visible to them, and they did not recognise Him, as is shown by Luke's narrative, until the breaking of the bread took place.
And thus, in apt accordance with the state of their minds, which were still ignorant of the truth, that it behooved Christ to die and rise again, their eyes sustained something of a similar order; not, indeed, that the truth itself proved misleading, but that they were themselves incompetent to perceive the truth, and thought of the matter as something else than it was. The deeper significance of all which is this, that no one should consider himself to have attained the knowledge of Christ, if he is not a member in His body— that is to say, in His Church— the unity of which is commended to our notice under the sacramental symbol of the bread by an apostle, when he says: “We being many are one bread and one body.” So was it that, when He handed to them the bread which He had blessed, their eyes were opened, and they recognised Him, that is to say, their eyes were opened for such knowledge of Him, in so far as the impediment was now removed which had prevented them from recognising Him.
For certainly they were not walking with closed eyes. But there was something in them which debarred them from seeing correctly what was in their view—a state of matters, indeed, which is the familiar result of darkness, or of a certain kind of humour. It is not meant by this, however, that the Lord could not alter the form of His flesh, so that His figure might be literally and actually different, and not the one which they were in the habit of beholding. For, indeed, even before His passion, He was transfigured on the mount so that His countenance “did shine as the sun.” And He who made genuine wine out of genuine water can also transform any body whatsoever in all unquestionable reality into any other kind of body which may please Him.
But what is meant is, that He had not acted so when He appeared in another form unto those two individuals. For He did not appear to be what He was to these men, because their eyes were holden, so that they should not know Him. Moreover, not unsuitably may we suppose that this impediment in their eyes came from Satan, with the view of precluding their recognition of Jesus. But, nevertheless, permission that it should be so was given by Christ on to the point at which the mystery of the bread was taken up. And thus the lesson might be, that it is when we become participants in the unity of His body, that we are to understand the impediment of the adversary to be removed, and liberty to be given us to know Christ.
Source: The Harmony of the Gospels (New Advent)