This is from page 44 of What’s So Great About the Doctrines of Grace? by Richard D. Phillips:
Were no one ever condemned–were there no display of God's judgment and wrath–there would be no knowledge of the glories of God's grace. In that case, the true God would be unknown to His creatures, and His purpose in creation–to display the fullness of His glory–would be unrealized. Having failed in this purpose, God would no longer be God. For this reason, God's decree of reprobation is necessary. God being perfect in every attribute, it is necessary for His every attribute to be exercised: goodness in creation, power in triumph, mercy in grace, and justice in wrath.
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