St. Thomas' Christology: Synthesis of De Deo Uno et Trino and De Homine

St. Thomas' Christology: Synthesis of De Deo Uno et Trino and De Homine

If you are a Thomistic philosopher without formal theological training, odds are you have not had a chance to study St. Thomas' IIIa Pars at any depth.  At best, as part of our Thomistic philosophical studies we philosophers have to read good chunks of the Summa theologiae, primarily select philosophically-themed questions, such as the text on the Five Ways and other questions on the One God (Ia, qq. 2-26), the questions on man (Ia, qq. 75 and following), good chuncks of the Ia-IIae on beatitude, human acts, on habits and virtues, and on law.  But rarely do we go out of those traditional loci Thomistici.  We hardly ever venture into the IIIa Pars, which is so clearly theological (although I would of course argue that the whole Summa is theological, through and through).    

But we philosophers should definitely read the IIIa Pars carefully.  In St. Thomas' Christological section of the IIIa Pars (qq. 1-59, esp. 1-26) we find so many of his philosophical doctrines coming into play in a marvellous way. This section is a true eye-opener for the philosopher: we can see the theological ‘mileage’ that St. Thomas gets out of his philosophical concepts. It is truly amazing how his philosophy of mind—which he discussed for its own sake in the Ia Pars—is now being applied perfectly to Christ's humanity in a way that it takes on new life.  

Further, this application of philosophical doctrines to Christ leads us to realize that in the Ia Pars he must have had Christ’s humanity in the back of his mind all along. The same can be said of his discussion of the virtues in the IIa pars (in particular the virtue of religion, with its discussion on prayer, and sacrifice). St. Thomas applies these questions so beautifully to his Christology, and therefore when writing the IIa Pars he must have anticipated what he was going to do here in the IIIa Pars.  I imagine he must have had all of these Christological applications in the back of his mind in those earlier sections because from the beginning it is evident that he had a very thoroughly detailed architectonic plan of what he wanted to do, so he must have thought of how the earlier sections applied to later sections from the beginning. 

Thus, the unity and harmony of the science of Sacred Theology, and of the mysteries of the faith that theology seeks to elucidate, really comes to light in this Christological section. Whereas before, in the Ia Pars he spoke of God in Himself, and then of man in himself (continuing on in the IIa Pars), here in the IIIa Pars he harmoniously applies both of those doctrinal treatises to the mystery of the God-man. Hence, Garrigou-Lagrange comments: 

Because the simpler things come before the composite… in the preceding parts of the Summa... what pertains to God and to man are discussed separately, whereas the present treatise is concerned with Him who is both God and man.¹

¹ Fr. Reginald Garrigou-Lagrange, OP, Christ the Savior, prologue. http://www.ewtn.com/library/theology/christ1.htm#00