76 But, again, a difficulty may also be felt in understanding how Mark says that the Lord appeared to the eleven as they sat at meat, if the time referred to is really the beginning of the night of that Lord's day, as is indicated by Luke and John. For John, indeed, tells us plainly that the Apostle Thomas was not with them on that occasion; and we believe that he left them before the Lord entered among them, but after the two disciples who returned from the village had been conversing with the eleven, as we discover from Luke.
Luke, it is true, presents a point in his narrative, at which we may fairly suppose, first, that Thomas went out while they were talking of these subjects, and then that the Lord came in. Mark, however, who says, “Lastly, He appeared unto the eleven as they sat at meat,” compels us to admit that Thomas also was there. But it may be the case, perhaps, that he chose to style them the eleven, although one of the company was absent, because the same apostolic society was designated by this number at the time previous to the election of Matthias in the place of Judas.
Or, if there is a difficulty in accepting this explanation, we may still suppose that, after the many manifestations in which He vouchsafed His presence to the disciples during the forty days, He also showed Himself on one final occasion to the eleven as they sat at meat—that is to say, on the fortieth day itself; and that, as He was now on the point of leaving them and ascending into heaven, He was minded on that memorable day specially to upbraid them with their refusal to believe those who had seen Him after He had risen until they should first have seen Him themselves; and this particularly because it was the case that, when they preached the gospel subsequently to His ascension, the very Gentiles would be ready to believe what they did not see.
For, after mentioning this upbraiding, Mark at once proceeds to subjoin this passage: “And He said unto them, Go ye into all the world, and preach the gospel to every creature. He that believes and is baptized shall be saved; but he that believes not shall be damned.” If, therefore, they were charged to preach that he who believes not shall be condemned, when that indeed which he believes not is just what he has not seen, was it not meet that they should themselves first of all be thus reproved for their own refusal to believe those to whom the Lord had shown Himself at an earlier stage until they should have seen Him with their own eyes?
Source: The Harmony of the Gospels (New Advent)