19. altar—not for sacrifice, but as the "pillar" for memorial and worship (Jos 22:22-26). Isaiah does not contemplate a temple in Egypt: for the only legal temple was at Jerusalem; but, like the patriarchs, they shall have altars in various places.
pillar—such as Jacob reared (Ge 28:18; 35:14); it was a common practice in Egypt to raise obelisks commemorating divine and great events.
at the border—of Egypt and Judah, to proclaim to both countries the common faith. This passage shows how the Holy Spirit raised Isaiah above a narrow-minded nationality to a charity anticipatory of gospel catholicity.