I predict the Catholic blogosphere will be abuzz with discussion of the new translation in the next three months. I have looked forward to this change since junior year at TAC, and am so glad to be getting back to a Missal as exalted as the one I loved for four years.
What is "dignum" is fitting, worthy, appropriate. "Iustum" is very similar: it implies commensurability, fittingness, what is due.1 The Church is wise, and would not include two synonyms. I posit that the two words are not redundant. My thesis: "iustum" is to "dignum" as politics is to ethics.
A polis (a ruler and those he rules) is an image of our relationship with God: God is a beneficent, provident ruler. He is so great a ruler that our debt is enormous; a holocaust of self is too little, although it is the most we can give. It is "iustum" for us to offer our entire hearts in thanks.
It is also "dignum," for a different reason. "Dignitas" belongs to persons2 by their nature, because they have what Aristotle called a "divine" power, the intellect.3 From what St. Thomas says about the origin of "person,"4 I gather that "dignitas" has always been associated with persons. So perhaps, "dignum" implies a fittingness between persons, while iustum implies a fittingness between parts of a political entity.
See how much the Church teaches us in just a few words? We completely surrender our hearts to God not only as subjects bound by justice; we do it as persons overflowing with gratitude for another person who has loved us so completely as to merit our whole heart. No citizen owes his king such a complete surrender; a different image is evoked by "dignum."
"Dignum" and "iustum" capture two images of our relationship with God. Both are helpful but neither is perfect. Together, they form a complex and fuller picture of how we are to act in each present moment: giving completely by gratitude to the holiest of Kings and the most perfect of Friends. I look forward to saying these words in November!
1 Summa Theologica II-II q. 57.
2 Boethius: "a person is an individual substance of a rational nature," quoted by St. Thomas, I q. 29 a. 1
3 Aristotle, De Anima, III
4 Summa Theologica I, q 29, a 3, obj 2, again quoting Boethius (De Duab. Nat.)