It's already been defined....it was long before Roman Catholicism showed up.
Besides, Roman Catholicism didn't finalize their canon until Trent.
I suggest you look into the era of Pope Damasus and the synods of Rome, Hippo, and Carthage, 1500 years before Reformation-era dissenters made all Christian creeds and canons controverial again.
Please look at the "Decretal of Gelasius", attributed to a synod convoked by Pope Damasus in the year 382 AD.. The other one you want to look at is the Canon of Innocent I, sent in 405 AD to a Gallican bishop in answer to his inquiry about the canon.
Both contain all the deuterocanonicals, without any distinction, and are identical with the catalogue of Trent.
You can see for yourself that the books identified as Scripture exhibited very substantial continuity: Trent merely recognized and certified more than a millennium worth of Christian liturgical practice.