The Christian doctrine of the Trinity, and in particular the theology of "the consubstantiality of the Son with the Father," is crucial to Stephen's understanding of himself and his art. In Scylla and Charybdis he advances a reading of Shakespeare's Hamlet that relies upon what Haines, when Mulligan tells him about the theory in Telemachus, calls “The Father and the Son idea. The Son striving to be atoned with the Father.” Mulligan mocks the Shakespeare argument in that opening chapter, and Stephen reciprocates by associating Mulligan with early Christian theologians whom the Catholic church views as heretics: "A horde of heresies fleeing with mitres awry: Photius and the brood of mockers of whom Mulligan was one, and Arius, warring his life long upon the consubstantiality of the Son with the Father, and Valentine, spurning Christ's terrene body, and the subtle African heresiarch Sabellius who held that the Father was Himself His own Son." All four of these thinkers advanced ideas about how God the Father relates to God the Son.

Earliest attested representation of the Scutum Fidei, a detail from an illustration to the Compendium Historiae in Genealogia Christi by Peter of Poitiers (Cotton Faustina ms. B. VII, folio 42v), ca. 1210. Source: Wikimedia Commons.

A modern version with the words written legibly. Source:Wikimedia Commons.

A more complicated diagram adding theological terms for the relationships that exist between the different Persons.  Source:

Chart diagramming the argument between the western and eastern churches. Source:

Detail from the Summa Vitiorum or Treatise on the Vices by William Peraldus (Harleian ms. 3244, folios 27-28), showing a knight allegorically shielded from dangerous sins by the Scutum Fidei. Source: Wikimedia Commons.