A.T. Robertson Commentary Luke 17:31

"In that day, he that shall be on the housetop, and his goods in the house, let him not go down to take them away: and let him that is in the field likewise not return back." — Luke 17:31 (ASV)

Let him not go down (μη καταβατω). Second aorist active imperative of καταβαινω with μη in a prohibition in the third person singular. The usual idiom here would be μη and the aorist subjunctive. See Mr 13:15f.; Matthew 24:17f. when these words occur in the great eschatological discussion concerning flight before the destruction of Jerusalem. Here the application is "absolute indifference to all worldly interests as the attitude of readiness for the Son of Man" (Plummer).