As noted in Chapters 2 and 5, fragments one and seven of Gildas's letter to Finnian are connected thematically by excommunication. This connection is further buttressed by the citing of both fragments as a continuous text in two early ninth-century Carolingian manuscripts – Vienna, Österreichische Nationalbibliothek, Lat. 2232 (V), and Munich, Bayerische Staatsbibliothek, Clm 14468 (B) – manuscripts used by Aidan Breen to compile his edition of the Incipiunt capitula canonica (as containing his edition of the Synodus II Patricii). This appendix combines the witness of the manuscript tradition (including that of Cambridge, Corpus Christi College, MS 279) with Flechner's edition of the Hibernensis, Elliot's edition of the Turonensis, and Breen's edition of the Incipiunt capitula canonica to merge and extend fragments one and seven into a new edition (and translation) concerning Gildas's approach to the unity of the Church – De communicatione Gildas.

Gildas's methodology in De communicatione follows that of the De excidio: a sequential scan for examples in the Old Testament (in this case, Genesis and Exodus), followed by parallel situations in the New Testament (in this case, Matthew and I Corinthians). Three new sources are identified as being part of Gildas's library: Ps-Clemens, Recognitiones (perhaps fourth century); Epistola Innocentii ad Exuperium Episcopum Tolosanum (ca 405); and Cyprian, De unitate (ca 250). For the purpose of this edition, Hibernensis and Turonensis are shortened to Hib and Tur respectively.

This new edition of the fragment from Gildas's letter to Finnian emphasises his criticism of those negantes in the church (potentially, as per Innocent, negantes veniam or Novatians) who excommunicate rather than denounce, particularly in the circumstances where guilt is not proven and where the crime is not a capital sin. It reveals an inclusive attitude to dealing with sin and penance, with the exclusion of a sinner only utilised as a last resort and in proven cases of major crimes.

This attitude on caution in the use of excommunication as laid out in this section of the letter to Finnian, as supported by the De excidio (where Gildas never calls for excommunication), stands in contrast to the active dependence on the disciplining powers of excommunication laid out in the Synodus I Patricii (sixteen of the thirty-four canons prescribe excommunication for a variety of non-capital transgressions), and in Patrick's Epistola (where Coroticus and his followers are excommunicated wholesale, without attention to individual guilt or innocence).