Article one sets out divine simplicity's philosophical motivation and its contemporary defenders and critics. Long before Plantinga and Stump argued over it in analytic journals, an ecumenical council had already made a version of the doctrine formal Catholic teaching, condemning a specific trinitarian error along the way.
The Fourth Lateran Council, in 1215, opened its constitution on the Catholic faith by affirming one true God, eternal, immeasurable, incomprehensible and ineffable, who is "one simple essence, substance or nature." The statement was directed against Joachim of Fiore, a Calabrian abbot who had accused Peter Lombard of dividing God into a kind of fourth entity by distinguishing the divine essence from the three persons too sharply. The council sided against Joachim and affirmed simplicity as the safeguard, one undivided substance the three persons share completely rather than a shared item standing apart from them, so the doctrine article one presents as chiefly a philosophical inference also carries the weight of a specific dogmatic condemnation.
The recent analytic debate article one reports, Brian Davies's contrast between classical theism and a rival increasingly called theistic personalism, which treats God as one person among others differing only in degree of power and knowledge rather than in kind of being, is in substance a continuation of that same thirteenth-century argument in modern vocabulary. Theistic personalists are not accused of Joachim's specific error, but the underlying question, whether God's attributes and persons are truly one simple reality or a collection of distinct items loosely unified, is the question Lateran IV closed for Catholic dogma and that philosophers of religion continue to reopen on independent grounds.