Matthew 24:2

‘But he answered and said to them, “Do you not see all these things? Truly I say to you, There will not be left here one stone upon another, that will not be thrown down.” '

So He informed them that, wonderful and ageless though the Temple may seem to be, (and all no doubt expected it to last for hundreds of years), there was coming a time when there would not be left one stone upon another, because God had rejected it. In other words, it would be torn down and wrecked, so that nothing was left of it. As we now know this destruction would be carried out by the Roman general Titus and his men about forty years later when in fact the Temple would be set on fire and burned, never to be rebuilt, with what remained of it finally disappearing below the ground.