Before 1945 the Gospel of Thomas was three scraps of Greek papyrus from Oxyrhynchus, published at the turn of the century and impossible to place: sayings attributed to Jesus, some familiar and some not, with no framework to fit them into. Codex II contains the whole thing in Coptic, one hundred and fourteen sayings, no narrative, no crucifixion, no resurrection, just a sequence of statements introduced by the words Jesus said.
That completeness reorganised a field. It made it possible to ask whether a sayings collection of this kind lay behind the canonical gospels, a question that had been theoretical, and it gave the study of Christian origins a text to argue over rather than a hypothesis.
The wider effect of the codex is broader still. Almost everything known about Gnostic Christianity before the find came from the men who wrote against it, Irenaeus and Hippolytus and Epiphanius, who quoted their opponents in order to refute them and who had no reason to quote them fairly. Nag Hammadi put the other side of that argument on the table. The Apocryphon of John, also in this codex, turns out to correspond closely to a system Irenaeus described, which is a rare piece of good news for his accuracy, and the Gospel of Philip turns out to be something his summaries had not prepared anyone for.