The Abomination of Desolation in Rome.
… let the reader understand.
The great tribulation of Jerusalem
‘So when you see the disastrous abomination, of which prophet Daniel spoke, set up in the Holy Place (let the reader understand), then those in Judaea must escape to the mountains; if a man is in the fields, he must not turn back to fetch his cloak. Alas for those which child, or with babies at the breast, when those days come! Pray that you will not have to escape in winter or on a Sabbath. For then there will be great distress such as, until now, since the world began, there has been, or ever will be again. And if that time had not been shortened, no one would have survived; but shortened that time shall be, for the sake of those who are chosen.
 cf. Mt 24: 15-25