Jamieson, Fausset and Brown on Jeremiah 51:50

50. escaped ... sword--namely, of the Medes. So great will be the slaughter that even some of God's people shall be involved in it, as they had deserved.

afar off--though ye are banished far off from where ye used formerly to worship God.

let Jerusalem come into your mind--While in exile remember your temple and city, so as to prefer them to all the rest of the world wherever ye may be (Is 62:6).