When it is explained in terms of relation, the idea of personhood enables one to put forward an authentic Trinitarian monotheism. The expression 'Trinitarian monotheism' is not part of Thomas' own way of speaking,1 but it represents what he is working toward: a plurality of persons who are one same and single God. That is why the first article of the Trinitarian treatise, studying the 'immanent' processions in order to side-step Arianism and Sabellianism, is written with this aim already in view, as are the studies of relation and person which follow it. The Summa Theologiae pictures this characteric feature of Christian faith from many complementary angles: as the 'plurality of persons' (q. 30), as the 'upshot' of unity and plurality in God (q. 31), and in terms of the 'comparison' of the persons with the common essence (q. 39).2
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