What sign will there be when the end is about to come? Jesus’ arrival in Jerusalem did not put a break in his ministry. He continued to teach the people and to answer the many questions that were put before him. In the Gospel passage that we have read today, the content of Jesus’ teaching was given in response to a remark that was made either by one of those who were part of Jesus’ cohort, or by one of the many people who visited the temple on a daily basis to be awed by its grandeur. Whereas Jesus must have not disputed the remark (the beauty of the temple), he did not want its grandeur to blind the eyes of the people to the fact that a time will come when it will cease to exist. And as if Jesus’ ‘revelation’ was not enough, it was the manner of its ceasing to be that caused an alarm. In the words of Jesus, the temple will face a destruction of an apocalyptic proportion ( there will not be left a stone upon another stone that will be not thrown down). To them, Jesus was speaking about the events of the end times because only such a calamity could lead to the destruction of their grandiose temple. Because of how they heard and ‘interpreted’ Jesus’ statement, they were thus justified in their concern: they wanted to know when such a dreaded event will take place in order not to be caught off-guard ( Teacher, when will this happen? And what sign will there be when all these things are about to happen?) 
In talking to those who were with him about the magnitude of the destruction that will befall the temple, Jesus was in a way bringing to the immediate future something that rightfully belonged to the remote future. From the apocalyptic literature with which they would have been familiar, those listening to Jesus were aware that the end was certain to come sometime in the unknown future. And due to the events/calamities that were understood to accompany and usher in the end-time, it was a dreaded time and no one wished that it takes place during his/her lifetime. Because of what the temple represented and how reverently it was regarded, anything that would lead to its destruction would not fall short of a catastrophe of the world coming to an end. It was thus that they wanted to know from Jesus when this would take place.
In his response, Jesus warned them (and us) to be on guard and not to let themselves be deceived by such events for the fabric of our world is partly made up of them. What one needs to do, as he tells us elsewhere, is to always be ready for the end by living as pilgrims, that is, as a people who know that this world is not our home and that it was not meant to exist forever. Jesus further warns us against people who will take advantage of such calamities in order to deceive us into whatever conniving schemes they have. He knows our fears, especially of facing our end, and that some of us will do anything to avoid that, even if it means selling one's soul. He tells us point blank that we can't avoid our end, that all we have to do is to live our lives in such a way that when the end finally arrives, we are ready to embrace it.