Thomas Aquinas’ treatise on the Trinity in the first part of his Summa Theologiae runs from question 27 to 43. It’s a vast discussion, all but comprehensive in its coverage of the most important elements of Trinitarian theology. So sweeping an account could start from any number of principles or points of view, but Thomas chooses a famously abstract opening consideration: Is there procession in God?
In courses on the Trinity, I assign as much Thomas as I think the students can handle, and I always include this opening text, which is Question 27, Article 1. Recently I made a short video about the article, just to show students how to read through it, to trace what Thomas is doing in his discussion, and to draw out a few of his presuppositions and implications.
In case this kind of running commentary might be helpful to more people, I’m sharing it here. The text of the Summa is freely and widely available, but for close reading purposes I also project all of it for the viewer to follow along.