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 (Gen 28:18; Gen 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.