On several counts, Thomas has for some time been criticized for the very structure of the Summa. (1) Christ comes last, “as though an afterthought”. (2) He treats the One God before he treates the Triune God. The objections are sometimes trivial and sometimes more profound. The profound objections hit their mark when Thomas is read / taught without that moment of synthesis (“so as to unite”). So great is the analytical rigor that we sometimes fail in bringing it all back together, as the Angelic Doctor intended.
Undoubtedly, what follows has been pointed out earlier (I am grateful for any references). Nevertheless, it may be worthwhile to draw attention to the matter here. The structure of the Summa resembles that of the Orthodox Faith by John of Damascus. Obviously, there is this similarity: in Part III John treats Christ. But consider also that John treats the existence, attributes, and unity of God (I, 3-5) before launching into Trinitarian considerations (I, 6ff). Granted, as does the NT, so John will use the term God to stand for the Father (see I, 6). Nonetheless, the treatment in I, 3-5) quite notably concerns the deity as such. In fact, there is an anticipation of this structure already in I, 2: “We … confess that God is without beginning, without end…. and that God is one… and that he is known and has his being in three subsistences” (NPNF, 9).
It is high time to return to the important contemplation of divine unity. And a robust unity it is. As Bruce Marshall indicates (The Thomist, 2010), Trinitarian theology is not worth its salt without it. This consideration brings out the importance both of continuity of NT with the OT and also the importance of Natural Theology.