Theophylact of Ohrid
Catena Aurea · Markusevangelium 13
Now learn a parable of the fig tree; When her branch is yet tender, and putteth forth leaves, ye know that summer is near: So ye in like manner, when ye shall see these things come to pass, know that it is nigh, even at the doors. Verily I say unto you, that this generation shall not pass, till all these things be done. Heaven and earth shall pass away: but my words shall not pass away.
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.