John Chrysostom

All these things therefore mean what was said of the end of Jerusalem, of the false prophets, and the false Christs, and all the rest which shall happen down to the time of Christ’s coming. That He said, This generation, He meant not of the men then living, but of the generation of the faithful; for so Scripture uses to speak of generations, not of time only, but of place, life, and conversation; as it is said, This is the generation of them that seek the Lord. (Ps. 24:6.) Herein He teaches that Jerusalem shall perish, and the greater part of the Jews be destroyed, but that no trial shall overthrow the generation of the faithful.