What is the earliest manuscript of the Gloria in excelsis Deo (Greater Doxology) hymn? What is the earliest manuscript of the Gloria in excelsis Deo (Greater Doxology) hymn?
According to Adrian Fortescue's The Mass: A Study in the Roman Liturgy, "§6. Gloria in Excelsis", p. 238, the Gloria in excelsis


  is found first [in Greek] in St. Athanasius' treatise: de Virginitate1 as part of morning prayer (with Ps. lxii, and the Benedicite) and in the [Greek Old and New Testament manuscript, the] Codex alexandrinus (Vth cent.).21. C. 20 (P.G. xxviii, 275). The authenticity of this work, long disputed, now seems more generally admitted. See Eichhorn : Athanasii de vita ascctica testimonia (Halle, 1886) pp. 27 seq., and especially von der Goltz in Texte und Untersuchungen, N.S. xiv, 2 a.2. As an appendix to the psalms at the end.


cf. also "Psalter" and my answer to the question "How did the Catholic Church function logistically/liturgically prior to the invention of the printing press?"