Charles Spurgeon Commentary Zechariah 13:7-8

"Awake, O sword, against my shepherd, and against the man that is my fellow, saith Jehovah of hosts: smite the shepherd, and the sheep shall be scattered; and I will turn my hand upon the little ones. And it shall come to pass, that in all the land, saith Jehovah, two parts therein shall be cut off and die; but the third shall be left therein." — Zechariah 13:7-8 (ASV)

So, in the times of God's fiercest judgments, he has a remnant according to the election of grace who shall escape the sword, because that sword has been awakened against him who was their Representative, their Surety, and who stood as Substitute in their place.