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Jeremiah 31:23 is an exciting and obscure passage on holiness. On the one hand, the passage talks about a promising future: a return of Israel from captivity to freedom, which was a foretaste of the redemption we have through Jesus Christ and in the earth made new. On the other hand, the passage uses cultural language that is very specific to ancient Judah and far removed from our daily ways of thinking and speaking.
Jeremiah 31:23 says, "Thus says the Lord of hosts, the God of Israel: 'They shall again use this speech in the land of Judah and in its cities, when I bring back their captivity: "The Lord bless you, O home of justice, and mountain of holiness!"'" (NKJV). What is exciting about this passage is the splendid imagery of a promised liberation: the people being back in their own land, inhabiting their own cities once again; getting to once more speak to each other in a well known greeting of praise; having their life situation no longer defined by captivity; and appealing beautifully to Jerusalem.
Jeremiah 31:23 is also filled with descriptions of God's closeness to his people, identifying him as the God of Israel and the Savior of Judah. And this exciting and fulfilling imagery makes the obscurity of this passage all the more tantalizing and intriguing. God's action of freeing his people from Babylonian captivity will cause them to once again make use of the blessing, "The Lord bless you, O home of justice, and mountain of holiness!" This expression is a reference to the city of Jerusalem, but it's also a part of Judean culture. Why is Jerusalem called a mountain of holiness? One of the obvious reasons is because Jerusalem was and is located on a large hill/small mountain; when going to Jerusalem, you literally have to go up to the city (which is why the Bible often says, "let us go UP to Jerusalem").
The other reason, however, is because people believed God graced Jerusalem with his presence: God had chosen Jerusalem as his city, so God would be present to the inhabitants of that city. In fact, the phrases "home of justice" and "mountain of holiness" are meant to convey the same thing - God is there with his people.
We can take encouragement from the fact that the God who frees his people from oppression and captivity is the same God who dwells with them and makes his holy and just presence known to them in love.