Joseph Addison Alexander on the jealousy/zeal of the Lord of hosts

"The word [translated "zeal"] expresses the complex idea of strong affection, comprehending or attended by a jealous preference of one above another. It is used in the Old Testament to signify not only God's intense love for his people but his jealousy in their behalf, that is to say, his disposition to protect and favor them at the expense of others. Sometimes, moreover, it included the idea of a jealous care of his own honor, or a readiness to take offence at anything opposed to it, and a determination to avenge it when insulted. There is nothing in this idea of the divine jealousy incongruous or unworthy. The expressions are derived from the dialect of human passion, but describe something absolutely right on God's part for the very reasons which demonstrate its absurdity and wickedness on man's. These two ideas of God's jealous partiality for his own people, and his jealous sensibility respecting his own honor, are promiscuously blended in the usage of the word, and are perhaps both included in the case before us. Both for his own sake and his people's, he would bring these events to pass. Or rather the two motives are identical, that is to say, the one includes the other. The welfare of the church is only to be sought so far as it promotes God's glory, and a zeal which makes the glory of the church an object to be aimed at for its own sake, cannot be a zeal for God, or is at best a zeal for God, but not according to knowledge. The mention of God's jealousy or zeal as the procuring cause of this result affords a sure foundation for the hopes of all believers. His zeal is not a passion, but a principle of powerful and certain operation. The astonishing effects produced by feeble means in the promotion, preservation, and extension of Christ's kingdom, can only be explained upon the principle that the zeal of the Lord of hosts has effected it."
-- Joseph Addison Alexander on Isaiah 9:7