A.T. Robertson Commentary Luke 21:20

"But when ye see Jerusalem compassed with armies, then know that her desolation is at hand." — Luke 21:20 (ASV)

Compassed with armies (κυκλουμενην υπο στρατοπεδων). Present passive participle of κυκλοω, to circle, encircle, from κυκλος, circle. Old verb, but only four times in N.T. The point of this warning is the present tense, being encircled. It will be too late after the city is surrounded. It is objected by some that Jesus, not to say Luke, could not have spoken (or written) these words before the Roman armies came. One may ask why not, if such a thing as predictive prophecy can exist and especially in the case of the Lord Jesus. The word στρατοπεδων (στρατος, army, πεδον, plain) is a military camp and then an army in camp. Old word, but only here in the N.T.

Then know (τοτε γνωτε). Second aorist active imperative of γινωσκω. Christians did flee from Jerusalem to Pella before it was too late as directed in Lu 21:21; Mr 13:14f.; Matthew 24:16f.