A.T. Robertson Commentary Mark 13:20

"And except the Lord had shortened the days, no flesh would have been saved; but for the elect`s sake, whom he chose, he shortened the days." — Mark 13:20 (ASV)

Whom he chose (ους εξελεξατο). Indirect aorist middle indicative. In Mark alone. Explains the sovereign choice of God in the end by and for himself.