In that day. When the Lord rises to seize the prey (Zephaniah 3:8), when the Gentiles are converted, and Judah returns to her obedience. Shalt thou not be ashamed for all thy doings. God addresses Israel repentant and converted, and assures her that she shall not have to reproach herself any more, or to blush for her iniquities, because God blots them out, or because she sins no more as she has done.
And the great help to this improvement is the abolition of the cause and incitement to sin. I will take away out of the midst of thee them that rejoice in thy pride (thy proud triumphers, Isaiah 13:3).
God will cut off all those who gloried in their temporal prosperity without thought of God, who in the pride of their heart walked as they pleased, deeming themselves accountable to no one, subject to no law.
Such shall no longer be found in the holy nation. Haughty because of (in) my holy mountain; i.e. in the temple (Isaiah 11:9). They shall no longer exult in the exclusiveness of their privileges, or feel a vain glorious confidence in their own election, or the sanctity of their temple or its provision of worship.
The Gentiles should be admitted to the covenant, and share in their privileges.