A third question is whether Melanchthon later gave up predestination; originally he had joined with Luther in decisively advocating it.
Note first that the refers to the universality of the promises of salvation and blames man’s debt on a miscarriage of the human will only, because God is not causa peccati, and in him is no contradictoriae voluntates. On the other hand, there seem to be assertions which do not fit within these limits. In the commentary on Romans in 1556 he answers the question, Why are so many men lost and so few saved, and why must the Church suffer in so many ways? “Not all the plans of God can be understood by human thought, but we must sustain ourselves in the revealed word of God… Someday in the eternal school we will learn the reason for the divine plan.” In this connection he often speaks of a hidden decree of God (arcanum dei decretum, consilium arcanum), or of the hidden majesty (arcana maiestas). In view of God’s decision with respect to Esau and Jacob, he declares that the same measure for measure is to be given only in the case of an obligation. “In the case of a gift or of compassion, it is not necessary to grant in the same measure.” “The sentence which says that there is an eternal election is true, and nevertheless it also remains true that we are not to investigate election without the word of God or beyond the word of God.” “We should agree to the word, even if we are not able to see all the connection in what is presented contradictorially” (quae se in contrarium offerunt). “You must know that you should not judge a priori about your election, but a posteriori, that is, you are not to search in the hidden counsel of God to discover whether you are elected, you are to search in the revealed word.”
These statements indicate that Melanchthon does not reject predestination in principle- even in the sense of reprobation, but admonishes practically and pastorally about it. This would mean that a responsible decision of man and a sovereign decision of God take place simultaneously in the acceptance or rejection of salvation. In this assertion of dual activity Melanchthon is speaking in a paradoxical way, as he does in the question about the relation of the acts of God and of man in the formation of faith.
Melanchthon, therefore, stands theologically nearer to Luther than the traditional view indicates. The important theological deficiencies of the time following Melanchthon are more the responsibility of students who fragmented what he had fused.
~Hans Engelland, Introduction to Loci Communes 1555 pg. xl-xli