In the final edition of the Loci communes (1559), Philip Melanchthon provides a good example of how to move from a philosophical to a theological definition of God. Melanchthon had added a section de Deo, missing in the first edition, to later editions of the Loci, but the passage below is not found in the second aetas, or “age,” of the work, but only in its third or final “age.”
He begins with a definition that he attributes to Plato: “God is the eternal mind, the cause of good in nature.” 1 He then unpacks this definition and indicates that it is fine as far as it goes. But it does not go far enough; for it does not describe God as he has revealed himself to be: it does not tell us about the Trinity. This only comes through the Word. Revelation does not destroy natural knowledge of God. But it adds the clarity of divine wisdom and doctrine that is not accessible to reason to what would otherwise be only “the thoughts of the human mind.” Those thoughts encapsulated in the Platonic definition are true, it should be noted; but they are incomplete.
In order that we may come to a definition of God, I will make a comparison of two of them. One definition is a truncated endeavor by Plato. The other is a complete one which has been handed down in the church and derives from the words of John the Baptist. Plato says, “God is the eternal mind, the cause of good in nature.” Now, though this definition by Plato is set up in so learned a way that it is difficult for those with little training to judge as to what is lacking, nevertheless, because it still does not describe God as He has revealed Himself, a clearer and more appropriate definition is required. The definition is: “God is an eternal mind, that is, a spiritual essence, intelligent, the eternal cause of good in nature, that is, a truthful, good, just, almighty Creator of all good things, of the whole order in nature, and of human nature, all of which are directed to a certain orderly goal, that is, obedience. Plato has included all of these things. But they are still the thoughts of the human mind which, even though they are true and learned and developed on the basis of sure and demonstrable evidence, nevertheless are in need of an addition to tell us what kind of God He has revealed Himself to be. Therefore we must turn to the second definition: God is a spiritual, intelligent essence, eternal, truthful, good, pure, just, merciful, free, immeasurably powerful and wise, the eternal Father who has begotten His Son from eternity as His own image; the Son who is the coeternal image of the Father; and the Holy Spirit who proceeds from the Father and the Son, as the Deity has been revealed in the sure Word; that the eternal Father with the Son and the Holy Spirit created and preserves the heaven and the earth and all creatures; and among the human race, which was created to be in His image and to be obedient to Him, He has chosen for Himself the church so that by this church the one and true Deity might be revealed with sure and certain witness through the Word which has been given by the prophets and apostles, so that He might be recognized, invoked, and worshiped according to that divinely given Word; and all religions should be condemned which devise other gods, and this true Deity should be glorified in eternal life. (Tr. JAO Preus [modified], in The Chief Theological Topics: Loci Praecipui Theologici 1559 [2nd ed.: Concordia, 2011], pp. 12-13; Latin text here.)