Jordan Ballor, in a post at TCI, notes that Luther used the Medieval formula facientibus quod in se est Deus non denegat gratiam in his mature theology, but in a manner different from the theology of the via moderna. There is a passage in Luther’s Lectures on Galatians which corroborates Ballor’s point:
“God does not require of any man That he do more than he really can.” This is actually a good statement, but in its proper place, that is, in political, domestic, and natural affairs. For example, if I, who exist in the realm of reason, rule a family, build a house, or carry on a governmental office, and I do as much as I can or what lies within me (quantum possum vel quod in me est), I am excused. For this realm has boundaries, and to this realm these statements like “to do what lies within one” (Facere quod in se est) or “to do as much as I can” (facere, quantum possum) properly apply. But the sophists drag these statements into the spiritual realm, where a man cannot do anything but sin, because he is “sold under sin” (Rom. 7:14). But in external matters, that is, in political and domestic affairs, man is not a slave but a lord of these physical matters (corporalium rerum). Therefore it was wicked of the sophists to drag these political and domestic statements into the church. For the realm of human reason (Regnum … rationis humanae) must be separated as far as possible from the spiritual realm (spirituali Regno). (WA, 40. I. Band, 2. Galatervorlesung [cap. 1 –4] 1531, p. 292-293; LW, 26:173-174).
This corresponds with Luther’s rejection of Aristotelian virtue as the paradigm for spiritual virtue or righteousness. An interesting thing to note here is Luther’s division between the two Regna or Kingdoms permits him to use a principle that he often appears to reject outrightly. Thus, the principle of facientibus quod in se est is only sinful if one attempts to use it in spiritual affairs or consider it a theological principle rather than one that solely denotes political action.