Far too many of the Jews of Jesus’ day focused attention on the temple itself, and many probably worshiped that building instead of God Himself. Jesus’ disciples pointed out to Him the temple along with that which Herod had built too. He told them that not one stone of the temple would be left on another, all would be torn down. Later the disiples asked Him when this would happen and what would be signs of it.
The “end of the age” mentioned in verse 3 does NOT mean the end of the world. The end which Jesus (accurately) predicts is the end of the Old Testament era and the sacrificial system, as well as judgment on those who in between would crucify Him.
His command to them, and which is definitely applicable to us, is “See to it that no one misleads you.” In order to be obedient to this command we (and they) must be alert, study the Word, and spend time with Him . . . as well as obey ALL His commands.
The Gospel WAS preached to all the known world before the destruction of Jerusalem in AD70. Are we holding up our end?