What makes the Church holy and eternal?

The days will come when there will not be left here one stone upon. (Luke 21: 6)

And while some were speaking of the temple, how it was adorned with noble stones and offerings, Jesus said, “As for these things that you see, the days will come when there will not be left here one stone upon another that will not be thrown down.” And they asked him, “Teacher, when will these things be, and what will be the sign when these things are about to take place?” And he said, “See that you are not led astray. For many will come in my name, saying, ‘I am he!’ and, ‘The time is at hand!’ Do not go after them. And when you hear of wars and tumults, do not be terrified, for these things must first take place, but the end will not be at once. Then he said to them, “Nation will rise against nation, and kingdom against kingdom. There will be great earthquakes, and in various places famines and pestilences. And there will be terrors and great signs from heaven.” (Luke 21: 5-11)

In order to avoid the same fate of the Temple of Jerusalem, which was destroyed according to Jesus' prophecy only a few decades after his death and resurrection, the new Israel, that is Jesus’s Church, needs be faithful to her Lord. Therefore, it is not by human efforts or the attractive ideas of fashionable theologians that the Church is made holy and eternal; Christians are simply asked to bear witness to Jesus as the world’s one and only Saviour.