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We can’t exhaustively define God, of course, but this description cited by Dallas Willard is edifying.
God is “the eternal, independent, and self-existent Being; the Being whose purposes and actions spring from Himself, without foreign motive or influence; he who is absolute in dominion; the most pure, the most simple, the most spiritual of all essences; infinitely perfect; and eternally self-sufficient, needing nothing that he has made; illimitable in his immensity, inconceivable in his mode of existence, and indescribable in his essence; known fully only by himself, because an infinite mind can only be fully comprehended by itself. In a word, a Being who, from his infinite wisdom, cannot err or be deceived, and from his infinite goodness, can do nothing but what is eternally just, and right, and kind.”
— Adam Clarke in Cyclopaedia, vol. 3 (New York: Harper & Brothers, 1894), 903-4, quoted by Dallas Willard in Knowing Christ Today, chapter 4, n. 1.