A succinct definition of holiness from Frederick Buechner:

ONLY GOD IS HOLY, just as only people are human. God’s holiness is God’s Godness. To speak of anything else as holy is to say that it has something of God’s mark upon it. Times, places, things, and people can all be holy, and when they are, they are usually not hard to recognize. [1]

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[1] Frederick Buechner, “Holiness,” in Beyond Words: Daily Readings in the ABC’s of Faith (New York: HarperCollins, 2004).