Them learned men would commit these teachings to the parchment..... Just like the early Christians.... After that, it must be taken on faith. Faith that God would protect his word from corruption. guided by the Holy Spirit.
This foundational presupposition can be demonstrated to be false immediately and irrevocably!
The earliest copy of a copy of a copy is an itsy bitsy postage stamp sized piece of confetti! Should we say the Holy spirit has "preserved" this??
The oldest existing bible copies (of copies of copies, etc) do not have any sort of provenance as to who/what/when/where/how they came about-----nor do they match regional copies (of copies of copies, etc.) from other locations at about the same time.
Without provenance no masterpiece of art could be taken on face value nor any antique because it is profitable to market forgeries and clever (and rich) are the ones dealing in such "antiquities".
Not to put too fine a point on it: the actual "preserved by holy spirit" manuscripts consist of shreds from landfills, viritual dumpsters, rotting and dismantled by corruption and the ravages of centuries.
A real Holy Spirit preservation might be expected to consist of less stench, rot, shredding and non-verification by provenance!
At the time of Constantine, his mother had quite a hobby of purchasing flagrant "holy relics" forgeries by con-men out to make a buck off the Emperor's family preoccupation with christianity. The idea that the "true cross" of Jesus could be found wholly preserved and owned as a spiritual treasure is both lunatic and naive!
These people were borderline nuts. And I doubt the border was well-guarded!