150 Again, what is given by Matthew in the following terms: “Then let them which be in Judæa flee into the mountains; and let him which is on the house-top not come down to take anything out of his house; neither let him which is in the field return back to take his clothes,” is reported also by Mark almost in so many words. On the other hand, Luke's version proceeds thus: “Then let them which are in Judæa flee to the mountains.” Thus far he agrees with the other two.
But he presents what is subsequent to that in a different form. For he goes on to say, “And let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto: for these be the days of vengeance, that all things which are written may be fulfilled.” Now these statements seem to present differences enough between each other. For the one, as it occurs in the first two evangelists, runs thus: “Let him which is on the house-top not come down to take anything out of his house;” whereas what is given by the third evangelist is to this effect: “And let them which are in the midst of it depart out.”
The import, however, may be, that in the great agitation which will arise in the face of so mighty an impending peril, those shut up in the state of siege (which is expressed by the phrase, “they which are in the midst of it”) will appear upon the housetop [or “wall”], amazed and anxious to see what terror hangs over them, or what method of escape may open. Still the question rises, How does this third evangelist say here, “let them depart out,” when he has already used these terms: “And when you shall see Jerusalem compassed with an army”?
For what is brought in after this— namely, the sentence, “And let not them that are in the countries enter thereinto”— appears to form part of one consistent admonition; and we can perceive how those who are outside the city are not to enter into it; but the difficulty is to see how those who are in the midst of it are to depart out, when the city is already compassed with an army. Well, may not this expression, “in the midst of it,” indicate a time when the danger will be so urgent as to leave no opportunity open, so far as temporal means are concerned, for the preservation of this present life in the body, and that the fact that this will be a time when the soul ought to be ready and free, and neither taken up with, nor burdened by, carnal desires, is imported by the phrase employed by the first two writers— namely, “on the house-top,” or, “on the wall”?
In this way the third evangelist's phraseology, “let them depart out” (which really means, let them no more be engrossed with the desire of this life, but let them be prepared to pass into another life), is equivalent in sense to the terms used by the other two, let him not come down to take anything out of his house (which really means, “let not his affections turn towards the flesh, as if it could yield him anything to his advantage then”). And in like manner the phrase adopted by the one, “And let not them that are in the countries enter thereunto” (which is to say, “Let not those who, with good purpose of heart, have already placed themselves outside it, indulge again in any carnal lust or longing after it”), denotes precisely what the other two evangelists embody in the sentence, “Neither let him which is in the field return back to take his clothes,” which is much the same as to state that he should not again involve himself in cares of which he had been unburdened.
Source: The Harmony of the Gospels (New Advent)