Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 13:28-31

“Or else, This generation shall not pass away, that is, the generation of Christians, until all things be fulfilled, which were spoken concerning Jerusalem and the coming of Antichrist; for He does not mean the generation of the Apostles, for the greater part of the Apostles did not live up to the destruction of Jerusalem. But He says this of the generation of Christians, wishing to console His disciples, lest they should believe that the faith should fail at that time; for the immoveable elements shall first fail, before the words of Christ fail; wherefore it is added, Heaven and earth shall pass away, but my words shall not pass away.”