John Calvin Commentary Zechariah 12:9

"And it shall come to pass in that day, that I will seek to destroy all the nations that come against Jerusalem." — Zechariah 12:9 (ASV)

The Prophet repeats again that, though ungodly and wicked men attacked the Church in great numbers on every side, God would still be its defender. By saying, I will seek to destroy, etc., he means that God would be fully intent (intentum) on destroying, as people are accustomed to be anxious when they earnestly pursue an object. So that the faithful would not think that they might perish through the disdain, neglect, or forgetfulness of God, he says that He would be their anxious defender. I will seek then, that is, I will be most earnestly intent, to destroy all the nations.

This promise no doubt extends far wider than to the Jews, for he prophesies here concerning the kingdom of Christ. For if we consider the state of the people during the whole of the intervening period, from their return to the coming of Christ, the Prophet will certainly appear to have given here a hope of something far greater than what had actually occurred. But he especially had Christ in view. So, a perpetual defense is promised here to the Church, and from this also arises confidence in salvation, for God carefully watches over us, so that He may effectively oppose all our enemies.

I only briefly touch on these things, which require long and detailed consideration, but it is enough for me to show briefly the meaning of the Prophet, provided this is done clearly, so that each person may then apply what is said for their own edification. Meanwhile, we may also learn from the words of the Prophet that the Church is always to be troubled in this world, for not only one enemy will cause trouble to it, but even many nations will rise up against it.