John Gill Commentary Zechariah 13:8

"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 13:8 (ASV)

And it shall come to pass, [that] in all the land, saith
the Lord
Either in all the land of Israel, as Kimchi interprets it; or rather in all the world, as Aben Ezra and others; for this prophecy seems to have respect, not to the calamities of the Jews at the destruction of Jerusalem, after the death of Christ, when a few were saved, a remnant according to the election of grace, yet so as by fire, for whose sake the days of tribulation were shortened; but to the external state of the church, and the trouble of it throughout Christendom, about the time of the destruction of antichrist:

two parts therein shall be cut off [and] die ;
all hypocrites, formalists, and outward court worshippers; who seem as if they would be at this time two thirds of the professors of true religion; who will not be able to stand the hour of temptation that will come upon all the earth, to try the inhabitants of it; which will be the last struggle of the beast of Rome, (Revelation 3:10) but will be twice dead, plucked up by the roots; will die to the profession of religion, and be cut off from the people of God, and have no more a name with them:

but the third shall be left therein ;
the few names in Sardis, which have not defiled their garments; the hundred forty and four thousand that will stand with Christ, and by him, on Mount Zion, being redeemed from among men, (Revelation 3:4) (Revelation 14:1Revelation 14:4) compare with this (Revelation 16:19) .