Whoever is a son must be an heir as well. For merely by being born he deserves to be an heir. No work and no merit brings him the inheritance, but only his birth., Thus he obtains the inheritance in a purely passive, not in an active way; that is, just his being born, not his producing or working or worrying, makes him an heir. He does not do anything toward his being born but merely lets it happen. Therefore we come to these eternal goods – the forgiveness of sins, righteousness, the glory of the resurrection, and eternal life – not actively but passively. Nothing whatever interferes here; faith alone takes hold of the offered promise. Therefore just as in society a son becomes an heir merely by being born, so here faith alone makes men sons of God, born of the Word, which is the divine womb in which we are conceived, carried, born, reared, etc. By this birth and this patience or passivity which makes us Christians we also become sons and heirs. But being heirs, we are free of death and the devil, and we have righteousness and eternal life. This comes to us in a purely passive way; for we do not do anything, but we let ourselves be made and formed as a new creation through faith in the Word.
Martin Luther Luther’s Works 392