Then; i.e. when they shall see "the abomination of desolation," etc. Them which be in Judaea. Not only in Jerusalem, but in its vicinity, as most exposed to danger from the invading army. Flee into ( ἐπι Ì, over) the mountains.
The Christians seem to have taken this advice when the city was attacked by Costius Gallus, about A.D. 66, some three or more years before the siege under Vespasian. Gallus had appeared before the walls, and apparently had every hope of taking the city, when, for some reason not certainly known (either owing to a supposed defeat, or ignorance of his own success, or the advice of his generals), he suddenly withdrew his forces (Josephus, 'Bell.
Jud.,' 2.19, 6, 7). The Christians, bearing Christ's warning in mind, and having, as we may conjecture, seen the predicted sign, took the opportunity of flight from the doomed city, and made their escape to Pella, a town of Decapotis, southeast of Bethshean, and the ruins of which are known now by the name of Fahil.
Euschius probably refers to this migration ('Hist. Eccl.,' 3.5), narrating that, owing to a certain revelation given to holy men among them, the whole body of the Church, before the war, removed across the Jordan to Pella, and dwelt there in safety during those troublous times.
We probably, however, do not know the exact time of the flight, as we are ignorant of what was the warning of imminent danger which rendered this hurried proceeding necessary.