As he sat on the Mount of Olives, the disciples came to him privately, saying, "Tell us, when will this be, and what will be the sign of your coming and of the close of the age?"
The "this" the apostles are asking about is the prophesied destruction of the Temple at Jerusalem ("Not one stone shall be left standing on another," said Jesus.) The apostles are naturally as eager to hear details as you or I would be about a prophesied annihilation of every Catholic altar in the world–and for similar reasons. For the Temple was The place of sacrifice in the Old Testament. No Temple, no sacrifice. It was devastating news. What could it mean but the end of the world? And indeed it was the end of a world–the world of the Mosaic Covenant. More than that, it was a foretaste of the coming judgement on the whole world. For the Temple is a microcosm of the Universe just as the Universe is a macrocosmic Temple. Just as the Temple of Christ's Body was destroyed and God brought new life from it, so the Temple of Jerusalem and (one day) the Temple of this Universe pass away so that God can bring new life into the world. It's the way of things.