God ensured that everything we need to know for salvation and growth in Christ, spiritual maturity, was written down for us.
Nothing else can be trusted, especially word of mouth, to be passed down faithfully, without corruption or danger of loss.
We have the manuscripts that date back from that time period, including ones like the Dead Sea Scrolls which verify the continued integrity and fidelity to Scripture that those who copied it down held to.
THEY knew how important it was, even when others dismiss it as being relatively unimportant.
****Nothing else can be trusted, especially word of mouth, to be passed down faithfully, without corruption or danger of loss.****
Christ promised the protection of the Holy Spirit for precisely this reason.