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Isaiah 63:18 says, "The people of thy holiness have possessed it but a little while: our adversaries have trodden down thy sanctuary" (KJV). This is another passage in Isaiah that is difficult to translate; we're not sure if the verse should be translated as saying, "they [God's people] tended to a small piece of God's sanctuary" or "they for a small amount of time tended God's sanctuary." Regardless of how exactly Isaiah 63:18 should be translated, it is evident that the verse is talking about the people of God initially experiencing unrestricted access to the Jerusalem temple, only to have that access taken away by other nations.
What is interesting about this verse, however, is that God's holiness is technically mentioned twice. Israel is called a specific name in this passage: "the people of your [God's] holiness." This name indicates that the identity of Israel is not their own; even though they are experiencing difficulties from the other nations, especially in the face of their neglect of God's ways and unfaithfulness to their covenant relationship with him, they are still called "the people of your holiness." God still claims them as his own - and makes his own holiness a significant part of their reality - despite the hardships they are experiencing or have brought upon themselves. Further, the nations themselves are potential recipients of God's holiness. While the temple in Jerusalem (or the sanctuary in heaven) might be in view here, the literal word used can be translated either "sanctuary" or "holiness."
Isaiah 63:18 might be trying to say that the nations have torn down God's temple; however, it might also be trying to say that the nations are fighting God's holiness, with the parenthetical suggestion being that God had already given his holiness to the other nations.
What is the point which Isaiah 63:18 is trying to make about holiness? It’s two-fold point is that God's holiness is meant for everyone! If you are a believer, then you are defined by God's holiness - even if you personally don't think about yourself in those terms. And if you aren't a believer, God is still extending his presence and holiness to you - which means God doesn't turn his back on you, even if you wish to turn your back on him. And I think that's fantastic news about the holiness of God.