Melanchthon’s answer to the cause of salvation was complex. He strongly emphasized human responsibility and exhorted his readers to not resist God’s grace, lest they be damned and that for their own fault. At key points, however, he affirmed that non-resistance to God is itself the gift of God’s electing mercy. In this way he could attribute all glory to God for each person’s salvation. Yet his ambiguous statements and frequent insertion of exhortations about the activity of the human will into doctrinal discussions left him open to charges of synergism. . . .

Melanchthon did not reject Luther’s doctrine of predestination entirely, but in attempting to defend the doctrine against charges of fatalism or determinisms Melanchthon so highlighted human responsibility that the sovereignty of divine grace receded into the background, and divine sovereignty in reprobation disappeared almost entirely.

Joel R. Beeke, Debated Issues in Sovereign Predestination: Early Lutheran Predestination, Calvinian Reprobation, adn Variations in Genevan Lapsarianism, 33, 35.