Third petition of the late 5th-century "Deprecatio Gelasii" (PL 101, cols. 560-562; Capelle, below, p. 136), "le fleuron des litanies anciennes". The "Deprecatio Gelasii" was a Western adaptation of an Eastern litany more ancient still. For a critical edition, see B. Capelle, "Le Kyrie de la messe et le pape Gélase," Révue Bénédictine 46 (1934): 136-138 (126–144), which indicates that this petition is present in 9th-century Paris 1153 ("le seul connu"), f. 48v-49r, but absent from the abridged version at Angelica B. 3.18, f. 213r. Cf. 2 Tim 2:15 ("recte tractantem verbum veritatis") and Eph 3:10 ("multiformis sapientia Dei").
I was put onto this by Paul de Clerck ("Improvisation et livres liturgiques: leçons d'une histoire," Communautés et liturgies 60 (1978): 117 (109-126)), who overlooks (at least here) the allusion to Eph 3:10, but notes that there was a falling away or impoverishment after this point (petition no. 3 of the Franco-Gallican litany or prayer of the faithful reads merely
"For our pastor and all his clergy, we implore you"(Paul de Clerck, La «prière universelle» dans les liturgies latines anciennes: témoinages patristiques et textes liturgiques, Liturgiewissenschaftliche Quellen und Forschungen 62 (Münster Westfalen: Aschendorff, 1977), 187-205), a falling away that necessitated the rise of liturgical books (the libelli, then sacramentaries, that were the outcome of "the great 6th- [and post 6th-]century movement of compilation" (118)), i.e. a recognition that liturgical improvisation needed guidance (rich models; "books for the [support of] liturgy" rather than rigidly prescriptive "liturgical books" (118)); that early improvisation (characteristic of "the whole of Christian antiquity" up through the end of the 4th century (112)) had been "informed, not informal" (111); "directed" (119), rather than spontaneous; guided "by  general structures",  outlines (even, sometimes, written outlines) of prayers, and  the prodigious retentive capacities of an oral culture (113). For these reasons it did not result in "The monotony of a malnourished liberty [(La monotonie d'une liberté mal nourrie)]" (Louis Bouyer, L'improvisation dans l'Eglise ancienne, 15, as quoted on p. 120) or any marked declination from orthodoxy (115 ff.).