This and the next several verses give specific instructions for what one must do to escape the catastrophe that is going to strike Judea and Jerusalem.

Then may be translated “When this happens” or “When you see this happen.”

Those who are in Judea is best taken to mean “those who live in Judea.” Judea was discussed in 2.1.

The expression of Revised Standard Version, let … flee to the mountains, is more clearly expressed by Good News Translation‘s “must run away to the hills.” “Run to the hills to escape” is also good. This is a command in the third person, not a granting of permission.

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .