Theophylact of Bulgaria
As if He had said, As when the fig tree puts forth its leaves, summer follows at once, so also after the woes of Antichrist, at once, without an interval, shall be the coming of Christ, who will be to the just as summer after winter, but to sinners, winter after summer.
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.