Sorry but I'm not connecting those dots at all. How can anything be just apart from God declaring it so? How can anything be right or wrong apart from God declaring it so? God is the only source of the ethical normative.
Justice is an aspect of God's essence --what He is. If you believe that God is eternal, which I'm sure you do, then God's Nature must be to exist, or Existence Itself.
The science which deals with the most abstract conceptions must, therefore, be the science of the most universal conceptions. Among our ideas the most universal are Being, and the determinations of it which are called transcendental, namely unity, truth, goodness, and beauty, each of which is coextensive with being itself, according to the formulas, "Every being is one", "Every being is true", etc.Since God is Being Itself, then He must be Goodness Itself.
Justice means to give to something what it is due. Since God's nature is Goodness, He cannot fail to give to something what it is due.
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The fact that God can't declare good what is evil, isn't a limitation on His Power. (That's what Muslim's believe.) It's only a limitation in a logical sense. Which being is better and more powerful, a being that cannot lie, or a being that can? A being that can only give to a thing what it is due, or a being that can't? Which being is really limited?
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Secondly, consider the Scriptural data. The Bible tells us that Jesus is Truth. Were Jesus to declare good, evil, He would be a liar. Moreover, God warns us, "woe to those who call evil, good, and good, evil."