So then faith cometh by hearing, and hearing by the word of God. Hearing, not reading. No, Paul understood the distinction perfectly well. Just look at words regarding women in churches. If any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord.
Corruption is only possible once the words have been written down.
See previous comment. As before the word is spoken, not written. For this cause also thank we God without ceasing, because, when ye received the word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the word of God, which effectually worketh also in you that believe. It is easier to understand how they could mistake it for the word of men if they heard it rather than read it. For it is sanctified by the word of God and prayer.