The Hebrew word for holiness is kadosh, which also means separate. In some sense, the realm of the holy is the realm set apart — the Sabbath that is kadosh from the week, or the couple bound in kiddushin, the rites of marriage, sharing a unique intimacy.
Yet holiness cannot be fully separate. For we are told God is kadosh, and God is both above all and yet in all; and we too are told to be kadosh, to attain the state of both distance and closeness, separation and embrace. Holiness involves goodness — one cannot be holy without being good — but it is more than goodness.
Holiness touches the realm beyond ourselves and brings it into this world. A mundane action charged with mission, passion and sanctity becomes holy. When Israel is called a holy nation and a light, it is a reminder both to separate and to bring the fruits of that separation to others. The idea of holiness is awesome, full of mystery and wonder; it exceeds our ability to comprehend. Yet it can be expressed in a simple blessing, an act of extraordinary kindness, a congregation elevated together in prayer. For a moment, we experience the presence of God, and call that moment holy.