Luther explains1 that “God said: ‘Let Us make man as an image like Us [cf. Genesis 1:26].” To Luther (and indeed to all other Christians): this ‘Us’ is “the Father, Son, and Holy Spirit” – with that pre-incarnate “Son” quite the Central Person in the Trine God.
Now then, continues Luther: “God created man upright [Ecclesiastes 7:29, cf. Romans 2:14]. This uprightness was natural, in body and soul.” Thus Adam first obeyed the Law of nature; that is, the Law of God the Son.
Luther further comments,2 on Ecclesiastes 7:29: “God has placed man into things; has given him certain works and a certain task…. God made man upright, straightforward…, and to be content [cf. the Tenth Commandment]…. But man forsakes this uprightness.”
