To me it appears probable, that the Prophet reproves the Idumeans, because they became arrogant, as it were, against the will of God, and in opposition to it, when, at the same time, they were confined to the narrow passes of mountains. It is said elsewhere, (Mal. 1: 2,) 'Jacob and Esau, were they not brethren?' "But I have given to you the inheritance promised to your father Abraham; I have transferred the Idumeans to mount Seir." Now it is less bearable, if any one be elated with pride, when his condition is not so honorable. I therefore think that the Idumeans are here condemned because they vaunted so much, and arrogated to themselves more than what was right, when they yet were contemptible, when their condition was mean and obscure, for they dwelt on mount Seir. But others think that the punishment, which was impending over them, is here denounced, "Lo, little have I made thee among the nations", and Jeremiah says, 'and contemptible among men'; he omits the two words, thou and exceedingly; he says only, 'and contemptible among men'. But as to the substance, there is hardly any difference. If then we understand that that nation was proud without reason, the sense is evident, that is, that they, like the giants, carried on war against God, that they vaunted themselves, though confined to the narrow passes of mountains. Though I leave to others their own free opinion, I am yet inclined to the former view, while the latter has been adopted nearly by the consent of all; and that is, that God was resolved forcibly to constrain to order those ferocious men, who, for no reason, and even in opposition to nature, are become insolent.
Seen in this light, the basic course of the prophecy is this. We open with rumors of war (v. 1); and God begins to speak to Edom (Idumea), saying that they will be made small (v. 2). The prophet then points out that the pride of Edom has led to self-deception (v. 3) which will be violently overturned (v. 4-9). The prophecy then goes into greater detail about the particular type of pride of which Edom was guilty (vv. 10ff.) and affirms God's moral providence, which comes with judgment against the proud (vv. 15-16) and salvation for the just (vv. 17ff.). The whole makes a very striking little sermon on pride, and will leave phrases ringing in your head:
For near is the day of the Lord for all the nations!
As you have done, so shall it be done to you,
your deed shall come back upon your own head....
There's more to Obadiah than meets the eye; and the book is worth more attention than it usually gets.