Isaiah 26 is Judah’s millennial song. The faithful remnant in Israel is looking back to the Lord’s dealings with them, vindicating them among the nations in the great tribulation, and looking forward to the prospect of Jerusalem as a city set apart to God and Israel as the first among the nations during Christ’s millennial reign.
“Open the gates,” cry the remnant, “that the righteous nation that keeps faith may enter in.”
That’s not a Christian sentiment, it’s a Jewish one, and it’s quite a change from what we are seeing today.