God could not cease being God even if sinners took it upon themselves to convict God of every evil known to humankind, to kill him, and then to declare that "God is dead!" By what or whom does God need justification? Does the pot say to the potter, Why have you made me thus? Nevertheless when a preacher comes with the words of forgiveness God wants the hearers to justify him for that act. That is what deum justificare means, to "give God his due." What is due God? Not a sacrifice or a work of the law, but trust in his promise alone.
Steven Paulson, Lutheran Theology