God's possession.

Herman Witsius has written:

“The godly therefore are God’s excellent possession, which he claims and preserves, and in which he boasts, as his crown of glory and royal diadem, Isa. lxii. 3. Which he esteems as his riches, and suffers not to become the property of another: and in this sense also may holiness by ascribed to them: a holy nation, a peculiar people, are joined together, 1 Pet. ii. 9."

(Witsius, Herman. The Economy of the Covenants Between God and Man, Volume II, Kingsburg, CA: den Dulk Christian Foundation, 1990, p. 4)